Remove unnecessary content_for? checks

We are checking whether we have any content before we yield content
blocks. This is unnecessary, because if the content block is empty,
it'll simply yield an empty string.

Furthermore it seems like the content block library_javascripts is
unnecessary can be deprecated in favor of page_specific_javascripts.

app/views/layouts/_head.html.haml

@@ -50,12 +50,10 @@
   = render 'layouts/startup_css'
   - if user_application_theme == 'gl-dark'
     = stylesheet_link_tag_defer "application_dark"
-    - if content_for?(:page_specific_styles)
     = yield :page_specific_styles
     = stylesheet_link_tag_defer "application_utilities_dark"
   - else
     = stylesheet_link_tag_defer "application"
-    - if content_for?(:page_specific_styles)
     = yield :page_specific_styles
     = stylesheet_link_tag_defer "application_utilities"
   - unless use_startup_css?
@@ -70,13 +68,9 @@
 
   = Gon::Base.render_data(nonce: content_security_policy_nonce)
 
-  - if content_for?(:library_javascripts)
-    = yield :library_javascripts
-
   = javascript_include_tag locale_path unless I18n.locale == :en
   = webpack_bundle_tag "sentry" if Gitlab.config.sentry.enabled
 
-  - if content_for?(:page_specific_javascripts)
   = yield :page_specific_javascripts
 
   = webpack_controller_bundle_tags

app/views/layouts/jira_connect.html.haml

@@ -7,7 +7,6 @@
     = stylesheet_link_tag 'https://unpkg.com/@atlaskit/reduced-ui-pack@10.5.5/dist/bundle.css'
     = javascript_include_tag 'https://connect-cdn.atl-paas.net/all.js'
     = javascript_include_tag 'https://unpkg.com/jquery@3.3.1/dist/jquery.min.js'
-    - if content_for?(:page_specific_styles)
     = yield :page_specific_styles
     = yield :head
   %body

app/views/projects/ci/lints/show.html.haml

 - page_title _("CI Lint")
 - page_description _("Validate your GitLab CI configuration file")
 - unless Feature.enabled?(:monaco_ci, default_enabled: true)
-  - content_for :library_javascripts do
+  - content_for :page_specific_javascripts do
     = page_specific_javascript_tag('lib/ace.js')
 
 %h2.pt-3.pb-3= _("Validate your GitLab CI configuration")