Make sure deleted object's entries are removed from object_types

[tomoasleep]

Description

Previously, YARD::RegistryStore#delete removes an object entry from @store but does not remove its entry from @object_types.
Therefore, some elements of YARD::Registry.all may be nil after an object deletion such as the following code.

$ pry
[1] pry(main)> require 'yard'
=> true
[2] pry(main)> YARD::Registry.register(foo = YARD::CodeObjects::ModuleObject.new(nil, :Foo))
=> #<yardoc module Foo>
[3] pry(main)> YARD::Registry.register(bar = YARD::CodeObjects::ModuleObject.new(nil, :Bar))
=> #<yardoc module Bar>
[4] pry(main)> YARD::Registry.all(:module)
=> [#<yardoc module Foo>, #<yardoc module Bar>]
[5] pry(main)> YARD::Registry.delete(foo)
=> #<yardoc module Foo>
[6] pry(main)> YARD::Registry.all(:module)
=> [nil, #<yardoc module Bar>]

This PR makes sure that YARD::RegistryStore#delete remove the deleted object's entry from @object_types to fix this behavior.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[coveralls]

Coverage decreased (-0.07%) to 93.699% when pulling 3a4c122 on tomoasleep:fix-registry-store-delete into 890d4ad on lsegal:master.

[lsegal]

Thanks for this PR!