Remove boilerplate allowed_push_host restriction

** Why are these changes being introduced:

* There is a boilerplate line in the gemspec file to restrict which
  sites can have releases pushed to them. We have not had this line in
  the past, and its presence appears to be part of the initial
  protections that need to be edited or removed.

** Relevant ticket(s):

* n/a

** How does this address that need:

* After considering whether to update this directive, or remove it
  entirely, it seems consistent with past practice to remove it entirely
  so this is what this PR does.

** Document any side effects to this change:

* None (we are free to consider implementing a real value for this, and
  reintroducing the directive, in a future ticket)

Author: matt-bernhardt

mitlibraries-theme.gemspec

@@ -9,10 +9,6 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/MITLibraries/mitlibraries-theme'
   spec.license       = 'MIT'
 
-  # Prevent pushing this gem to RubyGems.org. To allow pushes either set the "allowed_push_host"
-  # to allow pushing to a single host or delete this section to allow pushing to any host.
-  spec.metadata["allowed_push_host"] = "TODO: Set to 'http://mygemserver.com'"
-
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "https://github.com/MITLibraries/mitlibraries-theme"
   spec.metadata["changelog_uri"] = "https://github.com/MITLibraries/mitlibraries-theme/releases"