[NFC][analyzer][docs] Remove some orphaned images

Remove some images that were previously used on the old homepage of
Clang static analyzer (`clang-analyzer.llvm.org`) but became unused
since then.

Among the four removed images, `scan_build_cmd.png` was copied to a
different location within the repository (and the RST-based docs use it
from that location); while the other three are not referenced at all
(`git grep` produces no results).

Author: NagyDonat