Remove getCategories mechanism of specifying test categories

Summary:
This mechanism was mostly redundant with the file-based .categories
mechanism, and it was interfering with it, as any test which implemented
a getCategories method would not inherit the filesystem categories.

This patch removes it. The existing categories are preserved either by
adding a .categories file, or using the @add_test_categories decorator.

Reviewers: jingham, clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D39515

llvm-svn: 317277