CLN: replace unnecessary `freq_to_period_freqstr` with `PeriodDtype` constructor
xref #52064

replaced `freq_to_period_freqstr()` with `PeriodDtype(freq)._freqstr` in ValueError message in the definition of `_shift_with_freq()`
I replaced `freq_to_period_freqstr` with `PeriodDtype(obj)._freqstr / PeriodDtypeBase(obj._period_dtype_code, obj.n)._freqstr`. Now we don’t need the `function freq_to_period_freqstr`, which is why I removed it.

One comment:
In `pandas/core/arrays/period.py` in the definition of `raise_on_incompatible` I added
```
with warnings.catch_warnings():
    warnings.simplefilter("ignore")
```
because  otherwise the `test_maybe_convert_timedelta` (in `pandas/tests/indexes/period/test_period.py`) fail.

@MarcoGorelli, could you please take a look at this PR? Thanks @natmokval 