x86: remove unneeded defined(__ASSEMBLY__) check from asm/dwarf2.h

This header file has the following check at the top:

  #ifndef __ASSEMBLY__
  #warning "asm/dwarf2.h should be only included in pure assembly files"
  #endif

So, we expect defined(__ASSEMBLY__) is always true.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>