toolchain/gcc: cleanup gcc9 config option

This line should have been removed in 244847da "build: remove GCC9
support" but stayed in tree after an incomplete rebase. Fix it.

Signed-off-by: Paul Spooren <mail@aparcar.org>

toolchain/gcc/Config.in

@@ -9,9 +9,6 @@ choice
 	config GCC_USE_VERSION_8
 		bool "gcc 8.x"
 
-	config GCC_USE_VERSION_9
-		bool "gcc 9.x"
-
 	config GCC_USE_VERSION_10
 		bool "gcc 10.x"