gcc-plugins: drop support for GCC <= 4.7

Nobody was opposed to raising minimum GCC version to 4.8 [1]
So, we will drop GCC <= 4.7 support sooner or later.

We always use C++ compiler for building plugins for GCC >= 4.8.

This commit drops the plugin support for GCC <= 4.7 a bit earlier,
which allows us to dump lots of code.

[1] https://lkml.org/lkml/2020/1/23/545

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>

scripts/Kconfig.include

@@ -48,9 +48,6 @@ $(error-if,$(failure,command -v $(LD)),linker '$(LD)' not found)
 # Fail if the linker is gold as it's not capable of linking the kernel proper
 $(error-if,$(success, $(LD) -v | grep -q gold), gold linker '$(LD)' not supported)
 
-# gcc version including patch level
-gcc-version := $(shell,$(srctree)/scripts/gcc-version.sh $(CC))
-
 # machine bit flags
 #  $(m32-flag): -m32 if the compiler supports it, or an empty string otherwise.
 #  $(m64-flag): -m64 if the compiler supports it, or an empty string otherwise.