Fix flake8 'imported but unused' reports

$ flake8 | grep F401
./run_unittests.py:43:1: F401 'mesonbuild.mesonlib.is_linux' imported but unused
./mesonbuild/compilers/c.py:32:1: F401 '.compilers.CompilerType' imported but unused
./mesonbuild/compilers/cpp.py:23:1: F401 '.compilers.CompilerType' imported but unused

mesonbuild/compilers/c.py

@@ -30,7 +30,6 @@ from ..mesonlib import (
 from .c_function_attributes import C_FUNC_ATTRIBUTES
 
 from .compilers import (
-    CompilerType,
     get_largefile_args,
     gnu_winlibs,
     msvc_winlibs,

mesonbuild/compilers/cpp.py

@@ -21,7 +21,6 @@ from ..mesonlib import MesonException, version_compare
 
 from .c import CCompiler, VisualStudioCCompiler, ClangClCCompiler
 from .compilers import (
-    CompilerType,
     gnu_winlibs,
     msvc_winlibs,
     ClangCompiler,

run_unittests.py

@@ -42,7 +42,7 @@ import mesonbuild.modules.gnome
 from mesonbuild.interpreter import Interpreter, ObjectHolder
 from mesonbuild.mesonlib import (
     is_windows, is_osx, is_cygwin, is_dragonflybsd, is_openbsd, is_haiku,
-    is_linux, windows_proof_rmtree, python_command, version_compare,
+    windows_proof_rmtree, python_command, version_compare,
     BuildDirLock, Version
 )
 from mesonbuild.environment import detect_ninja