docs: Stop building the doc cache by default

Fixes gstreamer/gst-docs#36

docs/meson.build

@@ -28,7 +28,6 @@ configure_file(
 plugins_cache_generator = find_program(join_paths(meson.current_build_dir(), 'gst-plugins-doc-cache-generator'))
 plugins_cache = join_paths(meson.current_source_dir(), 'plugins', 'gst_plugins_cache.json')
 plugins_doc_dep = custom_target('build-doc-cache',
-    build_by_default: true,
     command: [plugins_cache_generator, plugins_cache, '@OUTPUT@', '@INPUT@'],
     input: plugins,
     output: 'gst_plugins_cache.json',
@@ -142,4 +141,4 @@ plugins_doc = [hotdoc.generate_doc('GStreamer-core-plugins',
     gst_c_sources: ['../plugins/elements/*.c', '../plugins/elements/*.h'],
     dependencies: [plugins_doc_dep],
     gst_cache_file: plugins_cache,
-)]
\ No newline at end of file
+)]