swscale/x86/swscale: Remove unused ASM constants

swscale/x86/swscale: Remove unused ASM constants

The last user of g15Mask, r15Mask, g16Mask and r16Mask was disabled
in 77a416e8aab77058b542030870fd7178b62d2a62 and finally removed in
36e8de07ed62609df45d064b56501e3084d25723; b15Mask and b16Mask were
apparently always unused (except for in_asm_used_var_warning_killer,
a function that only existed to make the compiler not optimize ASM
constants away).
w10 is unused since d604bab901f6dfaaad672ef2164e42b1f350474c, w02
since ef423a661818f3c0d8206a2abbc65ff555cc0c67.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>