BUG: NotFoundException from to_gbq when replacing table in project different from `project_id` parameter
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd

df = pd.DataFrame(
  {
    "A": pd.Series(1, index=list(range(2)))
  }
)

# 0) Start with an empty test_dataset. test_table does not exist at this point.


# 1) This successfully creates a table named test_table in dataset test_dataset in project willyn_data_project
df.to_gbq('willyn_data_project.test_dataset.test_table', project_id='willyn_job_project')

# 2) This fails with pandas_gbq.gbq.NotFoundException: Table does not exist
df.to_gbq('willyn_data_project.test_dataset.test_table', project_id='willyn_job_project', if_exists='replace')

```

#### Problem description
I am working on a project where researchers can write to a dataset in one GCP project, but must run query jobs in another. In the above code example, `willyn_data_project` is the name of the project that contains the dataset, and `willyn_job_project` is the name of the project where query jobs are executed.

When attempting to use `if_exists='replace'`, we get the error `pandas_gbq.gbq.NotFoundException: Table does not exist`. I believe what is happening is we are attempting to delete a table from `willyn_job_project`, when we should be deleting a table in `willyn_data_project`. 

#### Expected Output

No error.

#### Output of ``pd.show_versions()``

INSTALLED VERSIONS
------------------
commit           : 5f648bf1706dd75a9ca0d29f26eadfbb595fe52b
python           : 3.9.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.10.40-1rodete2-amd64
Version          : #1 SMP Debian 5.10.40-1rodete2 (2021-06-22)
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.2
numpy            : 1.19.5
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.4
setuptools       : 52.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : 0.15.0
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
