cmd/go: remove file created by test

The file cmd/go/testdata/src/testcache/script.sh was accidentally
committed with CL 83256. Sorry about that.

Updates #22593

Change-Id: Id8f07587ea97015ed75439db220560a5446e53e6
Reviewed-on: https://go-review.googlesource.com/83395
Reviewed-by: Russ Cox <[email protected]>

Author: ianlancetaylor