libstdc++: Remove redundant zeroing in std::bitset::operator>>= [PR113806]

libstdc++: Remove redundant zeroing in std::bitset::operator>>= [PR113806]

The unused bits in the high word are already zero before this operation.
Shifting the used bits to the right cannot affect the unused bits, so we
don't need to sanitize them.

libstdc++-v3/ChangeLog:

	PR libstdc++/113806
	* include/std/bitset (bitset::operator>>=): Remove redundant
	call to _M_do_sanitize.