chore(instr-mongodb): drop obsolete 'pretest' step from TAV run (#1998)

Since the move to npm workspaces in #1771 this pretest step is
no longer needed for TAV test runs.

Co-authored-by: Marc Pichler <[email protected]>

Author: trentm

plugins/node/opentelemetry-instrumentation-mongodb/.tav.yml

@@ -9,6 +9,3 @@ mongodb:
     - versions: ">=6 <6.4"
       node: '>=15.0.0'
       commands: npm run test-v5-v6
-
-  # Fix missing `contrib-test-utils` package
-  pretest: npm run --prefix ../../../ lerna:link