Remove property that re-computed microsecond
Just accessing the already-existing attribute is about 50-130x faster than re-computing it.  These results are from a 2011-era MBP.

Before:
```
In [1]: import pandas as pd
In [2]: ts = pd.Timestamp.now()
In [3]: %timeit ts.microsecond
The slowest run took 4.69 times longer than the fastest. This could mean that an intermediate result is being cached.
10000 loops, best of 3: 29.7 µs per loop
```

After:
```
In [1]: import pandas as pd
In [2]: ts = pd.Timestamp.now()
In [3]: %timeit ts.microsecond
The slowest run took 8.08 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 620 ns per loop
```

This accounts for the discrepancy discussed in #17234 (https://github.com/pandas-dev/pandas/pull/17297#pullrequestreview-57686247)

Per requests from @jreback, I'm refraining from moving forward with any other optimizations that this makes possible (e.g. removing the roundabout call to `convert_to_tsobject` from `Timestamp.to_pydatetime`).

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
