[lldb] Remove all 'clean' targets from test Makefiles

Summary:
To my knowledge we don't actually use or need these rules. And if we need them then
there is probably a better way to implement this than having all these random regexes.

Reviewers: labath, JDevlieghere

Reviewed By: labath, JDevlieghere

Subscribers: jingham, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74126