gdb_server: do not copy feature names for tdesc generation

gdb_server: do not copy feature names for tdesc generation

We always have feature names defined by string literals and the
standard guarantees static storage duration for them. Hence, there's
no need duplicating and then freeing them.

Valgrind-tested.

Change-Id: I1b77f966c548e3694141c63bd8680735f0f47505
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2028
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>