Description:
------------
We have a category of 'experimental' tests, driven by the file
'collections/default.experimental'.

It is unclear what the purpose of this category is, and in any case,
no work is done to move tests from 'experimental' to the normal test
suite. Also, the handling of 'experimental' clutters the collection
files.

Fix:
----
1. Removed the code for handling of experimental tests from MTR.

2. Removed 'collections/default.experimental' file.

Change-Id: I7d8db1df8be86741242191bfb42919e1c7a00e53