build: remove unused Meson import

Author: iinuwa

webext/add-on/meson.build

@@ -1,4 +1,3 @@
-fs = import('fs')
 zip = find_program('zip')
 
 addon_dir = datadir / 'credentialsd'
@@ -21,4 +20,4 @@ custom_target(
   ],
   install: true,
   install_dir: addon_dir,
-)
+)