build: --credential-file-override is no longer required (#635)

@chingor13 has configured our runtime environment, such that we no longer need to explicitly provide a credentials file.

How does this magic work, I didn't fully understand?

Source-Author: Benjamin E. Coe <bencoe@google.com>
Source-Date: Fri Aug 7 19:02:09 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 94421c47802f56a44c320257b2b4c190dc7d6b68
Source-Link: googleapis/synthtool@94421c4

Co-authored-by: Alexander Fenster <fenster@google.com>

packages/google-cloud-speech/synth.metadata

@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5936421202fb53ed4641bcb824017dd393a3dbcc"
+        "sha": "94421c47802f56a44c320257b2b4c190dc7d6b68"
       }
     }
   ],