libstdc++: [_Hashtable] Remove useless check for _M_before_begin node

When removing the first node of a bucket it is useless to check if this bucket
is the one containing the _M_before_begin node. The bucket before-begin node is
already transfered to the next pointed-to bucket regardeless if it is the container
before-begin node.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hahstable<>::_M_remove_bucket_begin): Remove
	_M_before_begin check and cleanup implementation.

Co-authored-by: Théo Papadopoulo <papadopoulo@gmail.com>