[jenkins] Coverage bot: Remove cross-project-tests

Directory coverage is recently enabled in https://reviews.llvm.org/D158703
The following file shows up at the top level because cross-project-tests is
also built in the coverage build:
`cross-project-tests/debuginfo-tests/llvm-prettyprinters/gdb/llvm-support.cpp`
This patch removes cross-project-tests from the project list.

Differential Revision: https://reviews.llvm.org/D159469