remove dependency on old standalone "mock"

[a-detiste]

this codebase already uses the newer "unittest.mock" from the standard library everywhere else

• No new new dependencies are required

--> it's actually the reverse here

[joachimmetz]

Thanks for the proposed changes, given plaso uses l2tdevtools to generate dependency files, I've made comparable changes using l2tdevtools in #5038

Closing this PR in favor of 5038

[a-detiste]

Thanks