kbuild: remove wrapper files handling from Makefile.headersinst

scripts/Makefike.headersinst creates asm-generic wrappers by itself
because scripts/Makefile.asm-generic created some of exported wrappers
outside uapi directories.

Now this distortion has been fixed.  scripts/Makefile.headersinst can
simply copy wrappers created by scripts/Makefile.asm-generic.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>