BUG: `to_xml` raises `KeyError` for `index=False` when the index does not start at zero
- [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.

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

---

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

```python
import pandas as pd

records = {"col1": [1, 2], "col2": [3, 4]}
df = pd.DataFrame(data=records)

df.index = range(1, 3)  # if this line is commented out it works as expected
xml = df.to_xml(index=False)
print(xml)
```

#### Problem description

This code snipped should not raise a `KeyError`. The index should be ignored regardless of its content.

Traceback:

```
Traceback (most recent call last):
  File "/Users/xxx/xml.py", line 7, in <module>
    xml = df.to_xml(index=False)
  File "/Users/xxx/pandas/core/frame.py", line 2967, in to_xml
    xml_formatter = TreeBuilder(
  File "/Users/xxx/pandas/io/formats/xml.py", line 458, in __init__
    self.handle_indexes()
  File "/Users/xxx/pandas/io/formats/xml.py", line 199, in handle_indexes
    x for x in self.frame_dicts[0].keys() if x not in self.orig_cols
KeyError: 0
```

#### Expected Output

```xml
<?xml version='1.0' encoding='utf-8'?>
<data>
  <row>
    <col1>1</col1>
    <col2>3</col2>
  </row>
  <row>
    <col1>2</col1>
    <col2>4</col2>
  </row>
</data>
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : f00ed8f47020034e752baf0250483053340971b0
python           : 3.9.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.5.0
Version          : Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.0
numpy            : 1.21.0
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.3
setuptools       : None
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : 4.0.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.06.1
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 4.0.1
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.4.15
tables           : None
tabulate         : 0.8.9
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
None

</details>
