Remove _initialize_valarith

Remove _initialize_valarith

I noticed that _initialize_valarith is empty.  This patch removes it.
Because init.c is constructed at build time, there's no reason to keep
empty initialization functions around, because there's no overhead to
reintroducing them when needed.

gdb/ChangeLog
2019-05-02  Tom Tromey  <tromey@adacore.com>

	* valarith.c (_initialize_valarith): Remove.