x86-64: Remove sysdeps/x86_64/fpu/s_cosf.S

On Ivy Bridge, bench-cosf reports performance improvement:

           s_cosf.S      s_cosf.c       Improvement
max        114.136       82.401            39%
min        13.119        11.301            16%
mean       22.1882       21.8938           1%

	* sysdeps/x86_64/fpu/s_cosf.S: Removed.