test_extensions: sync meson.build with Makefile.

Makefile does not specify ENCODING, meson.build should not,
either. Oversight in commit 877bf52cff.

Discussion: https://postgr.es/m/ZKYpvvNQdbQuRDGx@paquier.xyz
Reported-by: Michael Paquier

src/test/modules/test_extensions/meson.build

@@ -47,6 +47,6 @@ tests += {
       'test_extensions',
       'test_extdepend',
     ],
-    'regress_args': ['--no-locale', '--encoding=UTF8'],
+    'regress_args': ['--no-locale'],
   },
 }