With USE_SYSTEM_NCNN this seemingly(?) tries to check if glslang
libraries exists only to know if system ncnn has support which is
already ensured through DEPEND here.

And then the checks fail[1] with modern glslang, so yank out the
unnecessary block.

[1] https://bugs.gentoo.org/922512
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -55,3 +55,3 @@
 
-if(USE_SYSTEM_NCNN)
+if(0)
     set(GLSLANG_TARGET_DIR "GLSLANG-NOTFOUND" CACHE PATH "Absolute path to glslangTargets.cmake directory")