chore(bigquery): remove deprecated proto pkg

[alvarowolfx]

[subfuzion]

@alvarowolfx Kokoro CI is failing with the following reason:

        	decoding userfn fn:{fn:{name:"github.com/GoogleCloudPlatform/golang-samples/dataflow/flex-templates/wordcount.formatFn"  type:{kind:FUNC  parameter_types:{kind:STRING}  parameter_types:{kind:INT}  return_types:{kind:STRING}}}}  opcode:"ParDo"  inbound:{kind:MAIN  type:{type:{kind:SPECIAL  special:KV}  components:{type:{kind:STRING}}  components:{type:{kind:INT}}}}  outbound:{type:{type:{kind:STRING}}}
        bad function
        	caused by:
        	decoding DoFn fn:{name:"github.com/GoogleCloudPlatform/golang-samples/dataflow/flex-templates/wordcount.formatFn"  type:{kind:FUNC  parameter_types:{kind:STRING}  parameter_types:{kind:INT}  return_types:{kind:STRING}}}
        failed to decode userfn
        	caused by:
        github.com/GoogleCloudPlatform/golang-samples/dataflow/flex-templates/wordcount.formatFn not found. Register DoFns and functions with the the beam/register package.
--- FAIL: TestWordCount (0.10s)
FAIL
FAIL	github.com/GoogleCloudPlatform/golang-samples/dataflow/flex-templates/wordcount	0.120s

[telpirion]

Hey @alvarowolfx -- would you please resolve the merge conflicts in this PR? Once we have those addressed, let's merge this. Thanks!

[alvarowolfx]

Hey @alvarowolfx -- would you please resolve the merge conflicts in this PR? Once we have those addressed, let's merge this. Thanks!

@telpirion updated the PR with the latest changes.