remove x86 in BUILD.gn (flutter#170965)

based on [this
comment](flutter#170191 (review))
Towards flutter#170142

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Author: AbdeMohlbi

engine/src/build/toolchain/linux/BUILD.gn

@@ -134,23 +134,6 @@ gcc_toolchain("clang_x86") {
   is_clang = true
 }
 
-gcc_toolchain("x86") {
-  prefix = ""
-  asm = "${prefix}gcc"
-  cc = "${compiler_prefix}${prefix}gcc"
-  cxx = "${compiler_prefix}${prefix}g++"
-
-  readelf = "${prefix}readelf"
-  nm = "${prefix}nm"
-  ar = "${prefix}ar"
-  ld = "${link_prefix}${prefix}g++"
-  strip = "${prefix}strip"
-
-  toolchain_cpu = "x86"
-  toolchain_os = "linux"
-  is_clang = false
-}
-
 gcc_toolchain("clang_x64") {
   prefix = rebased_clang_dir
   asm = "${prefix}/clang"