remove pointless assignment

Presumably this was supposed to be regname[sizeof (regname) - 1] but was typoed
to regname[sizeof (rename) - 1].  However that should be unnecessary because
sprintf should null terminate.  As is this assignment is invalid ISO C because
rename refers to the function rename (), and sizeof on functions is undefined.
In GNU C C the size of functions is 1 so the expression is the same as
regname[0].  The following call to sprintf () clearly will over right that, so
the statement either has no effect or is invalid.  Given that it seems safe to
just remove it.  While we are there correct the size of regname, and switch
from snprintf to sprintf since we know the exact length of the result.

gas/ChangeLog:

2016-04-15  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-mips.c (md_begin): Remove useless assignment.