Don't cast away const in find_and_open_source

find_and_open_source casts away const, but hasn't needed to in a
while.  This removes the cast and a strangely hostile comment.

gdb/ChangeLog
2019-05-17  Tom Tromey  <tromey@adacore.com>

	* source.c (find_and_open_source): Remove cast.
2 files changed