Revert "Add LLVM_CONFIG support"

This reverts commit 2c1ac8ac1ddb532cf498cc97a5bb36d5a05b4094.
diff --git a/mesonbuild/envconfig.py b/mesonbuild/envconfig.py
index 97f7264..0e47c05 100644
--- a/mesonbuild/envconfig.py
+++ b/mesonbuild/envconfig.py
@@ -132,7 +132,6 @@
     'pkg-config': 'PKG_CONFIG',
     'make': 'MAKE',
     'vapigen': 'VAPIGEN',
-    'llvm-config': 'LLVM_CONFIG',
 }
 
 # Deprecated environment variables mapped from the new variable to the old one