meson: Remove redundant env set operation

This is immediately overriden 4 lines later, and starting with Meso
0.50.1 this will lead to a noisy warning.

tests/check/meson.build

@@ -145,7 +145,6 @@ foreach t : base_tests
     )
 
     env = environment()
-    env.set('GST_PLUGIN_PATH_1_0', meson.build_root())
     env.set('GST_PLUGIN_SYSTEM_PATH_1_0', '')
     env.set('CK_DEFAULT_TIMEOUT', '20')
     env.set('GST_STATE_IGNORE_ELEMENTS', '')