highway: Drop forcing gcc-only toolchain

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-extended/highway/highway_1.2.0.bb

@@ -17,5 +17,3 @@ EXTRA_OECMAKE = "-DBUILD_TESTING=0 -DCMAKE_BUILD_TYPE=Release"
 CXXFLAGS:append:arm = " -mfp16-format=ieee"
 # Option not supported with clang and its default format for __fp16 anyway with clang
 CXXFLAGS:remove:toolchain-clang = "-mfp16-format=ieee"
-
-TOOLCHAIN = "gcc"