Compiler Attributes: remove GCC 5.1 mention

GCC 5.1 is now the minimum version.
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

include/linux/compiler_attributes.h

@@ -104,7 +104,6 @@
 #define __deprecated
 
 /*
- * Optional: only supported since gcc >= 5.1
  * Optional: not supported by clang
  * Optional: not supported by icc
  *