Remove obsolete assertion from regcache.c

Remove obsolete assertion from regcache.c

When building I got:

../../binutils-gdb/gdb/regcache.c:935:24: error: the address of ‘ssize_t read(int, void*, size_t)’ will never be NULL [-Werror=address]

This happens because "read" used to be a parameter to this function,
which was then removed; but the assertion wasn't updated.

I don't think the assertion is relevant any more, to this removes it.
I'm checking it in as obvious.

2017-10-17  Tom Tromey  <tom@tromey.com>

	* regcache.c (regcache::xfer_part): Remove assertion.