[SCons] Remove in-tree-build flag from pip

This flag doesn't work with pip versions older than 21.1, but Cantera
builds fine without it.

The indicated behavior becomes default in 21.3 anyway, and then starts
generating warnings of its own in pip 22.0.

Fixes #1269

interfaces/cython/SConscript

@@ -107,7 +107,7 @@ ext = localenv.LoadableModule(f"cantera/_cantera{module_ext}",
                               obj, LIBPREFIX="", SHLIBSUFFIX=module_ext,
                               SHLIBPREFIX="", LIBSUFFIXES=[module_ext])
 
-build_cmd = ("$python_cmd_esc -m pip wheel -v --no-build-isolation --use-feature=in-tree-build --no-deps "
+build_cmd = ("$python_cmd_esc -m pip wheel -v --no-build-isolation --no-deps "
              "--wheel-dir=build/python/dist build/python")
 plat = info['plat'].replace('-', '_').replace('.', '_')
 wheel_name = (f"Cantera-{env['cantera_version']}-cp{py_version_nodot}"
@@ -172,7 +172,7 @@ if env["stage_dir"]:
 
     install_cmd.append(f"--root={stage_dir.resolve()}")
 
-install_cmd.extend(("--no-build-isolation", "--use-feature=in-tree-build", "--no-deps", "-v", "--force-reinstall",
+install_cmd.extend(("--no-build-isolation", "--no-deps", "-v", "--force-reinstall",
                     "build/python"))
 if localenv['PYTHON_INSTALLER'] == 'direct':
     mod_inst = install(localenv.Command, 'dummy', mod,