main/gcc6: delete in favor of Alpine's gcc6

Alpine Linux ships their own GCC6 package now (side by side with GCC8):
https://git.alpinelinux.org/cgit/aports/commit/?id=f76673fb030820b2053cad7e5b81ece31079d4cc

main/gcc6/003_all_default-fortify-source.patch

@@ -1,40 +0,0 @@
-# DP: Turn on -D_FORTIFY_SOURCE=2 by default for C, C++, ObjC, ObjC++,
-# DP: if the optimization level is > 0
-
----
- gcc/doc/invoke.texi          |    6 ++++++
- gcc/c-family/c-cppbuiltin.c  |    3 +
- 2 files changed, 9 insertions(+), 0 deletions(-)
-
-Index: b/gcc/doc/invoke.texi
-===================================================================
---- a/gcc/doc/invoke.texi
-+++ b/gcc/doc/invoke.texi
-@@ -7840,6 +7840,12 @@ also turns on the following optimization
- Please note the warning under @option{-fgcse} about
- invoking @option{-O2} on programs that use computed gotos.
- 
-+NOTE: In Alpine Linux, @option{-D_FORTIFY_SOURCE=2} is
-+set by default, and is activated when @option{-O} is set to 2 or higher.
-+This enables additional compile-time and run-time checks for several libc
-+functions.  To disable, specify either @option{-U_FORTIFY_SOURCE} or
-+@option{-D_FORTIFY_SOURCE=0}.
-+
- @item -O3
- @opindex O3
- Optimize yet more.  @option{-O3} turns on all optimizations specified
-Index: b/gcc/c-family/c-cppbuiltin.c
-===================================================================
---- a/gcc/c-family/c-cppbuiltin.c
-+++ b/gcc/c-family/c-cppbuiltin.c
-@@ -1176,6 +1176,10 @@ c_cpp_builtins (cpp_reader *pfile)
-   builtin_define_with_value ("__REGISTER_PREFIX__", REGISTER_PREFIX, 0);
-   builtin_define_with_value ("__USER_LABEL_PREFIX__", user_label_prefix, 0);
- 
-+  /* Fortify Source enabled by default for optimization levels > 0 */
-+  if (optimize)
-+    builtin_define_with_int_value ("_FORTIFY_SOURCE", 2);
-+
-   /* Misc.  */
-   if (flag_gnu89_inline)
-     cpp_define (pfile, "__GNUC_GNU_INLINE__");