Remove calls to taskInit() and taskExit() from the start and end of on blocks

[Reviewed by Sung]

We don't use these in any of the locale models and they are not necessary.
Remove the compiler code that inserts the calls, remove the compiler flag to
enable/disable inserting these calls, and remove the function definitions from
the modules.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21877 3a8e244f-b0f2-452b-bcba-4c88e055c3ca