Remove the system-api-stubs-docs target

Nothing consumes the output of this module anymore. The txt files
themselves will be removed in a follow-up change (it is easier after
this build rule stops them changing all the time).

Bug: 171029603
Test: m checkapi with txts removed
Change-Id: Iac487dd6c62504b33cf4e372ba4a7cf9808bf5fc