Fix clean InProcessTethering fail problem

CleanSpec only respect root of a git project.
Remove packages/Tethering/CldanSpec.mk because it never
be read.

Bug: 144320246
Test: manual build InProcessTethering and observed it is removed
      after adding new clean step.

Change-Id: I58750e4a1fe11018d83da1ebf828d1b3134ec765
Merged-In: I58750e4a1fe11018d83da1ebf828d1b3134ec765