[Test] Fully remove config.swift_host_lib_dir.

Somehow #68858 managed to remove `config.swift_host_lib_dir` without
removing the corresponding `config.substitutions.append`.  Fix.

rdar://116112040

Author: al45tair

test/lit.cfg

@@ -582,7 +582,6 @@ else:
 
 config.substitutions.append( ('%llvm_obj_root', config.llvm_obj_root) )
 config.substitutions.append( ('%swift-lib-dir', config.swift_lib_dir) )
-config.substitutions.append( ('%swift-host-lib-dir', config.swift_host_lib_dir))
 config.substitutions.append( ('%swift-libexec-dir', config.swift_lib_dir) )
 config.substitutions.append( ('%swift-plugin-dir', config.swift_plugin_dir) )
 config.substitutions.append( ('%llvm_src_root', config.llvm_src_root) )