remove rt_intelllvm.conf, rt_35d and rt_weekly

[DeniseWorthen]

Description

The LLVM compiled tests are now part of the standard RT suite, so rt_intelllvm.conf can be removed. This could be done as part of any PR I believe.

[gspetro-NOAA]

Do you think there's any value to keeping it in for now in case developers want to run only LLVM cases?
My sense is no because not all LLVM cases work on all platforms, so they'd probably have to change the file significantly anyway, but I figured I'd see if anyone can think of a use case.
I agree that it should be removed after a full switch to LLVM compilers on all supported systems.

[DeniseWorthen]

I think rt_conf.intelllvm just clutters up the test directory (along w/ rt_35d and rt_weekly). All can be removed.

A new weekly test will hopefully get re-instituted soon, but w/ a completely different set of platforms and/or tests, so the current one is really of no use.

[DusanJovic-NOAA]

I added rt_intelllvm.conf in #2224 last year, when I was adding initial porting to new Intel compilers. It is (was) just a copy of rt.conf in which I removed all gnu tests, and literally just search-replace 'intel' with 'intlelllvm'. Nothing else. I'm pretty sure nobody is using it today for anything, and it's probably out of date compared to main rt.conf.

It can be, and it should be, deleted. As well as two other .conf files.