Remove ExecuteOnBrowser wrapper on destroy

[zavitax]

Description

Remove ExecuteOnBrowser() wrapper from BrowserSource::DestroyBrowser().

Motivation and Context

Under certain conditions, when browser source scene items are removed
in a loop, CEF UI thread may hang waiting for previous invocations of
ExecuteOnBrowser() to complete resulting in a deadlock.

Since CefBrowserHost::WasHidden() and CefBrowserHost::CloseBrowser()
are not required to be called on a certain thread, removing
ExecuteOnBrowser() altogether preserves the functionality, eliminates
the deadlock and simplifies the code.

How Has This Been Tested?

• Remove single browser source
• Remove multiple browser sources
• Change scene collection
• Hide/showbrowser source with "Shutdown when hidden" checked
• Exit program

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.

[PatTheMav]

@WizardCM Can probably be closed with #506 merged.

[RytoEX]

Closing per above.