samples: update synth.py to exclude samples.yaml

[anguillanneuf]

This is to prevent synth from proposing PRs like #533. The synth-proposed change will cause the samples checkstyle test to miss this file https://github.com/googleapis/java-pubsub/blob/master/samples/checkstyle-suppressions.xml and therefore fail the checkstyle test.

The checkstyle exclusions are for classes generated by Avro tools and protoc. They don't comply to the strict checkstyle rules.

• https://github.com/googleapis/java-pubsub/blob/master/samples/snippets/src/main/java/utilities/State.java
• https://github.com/googleapis/java-pubsub/blob/master/samples/snippets/src/main/java/utilities/StateProto.java

[codecov]

Codecov Report

Merging #536 (598f4cc) into master (6f172ae) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #536   +/-   ##
=========================================
  Coverage     78.79%   78.79%           
  Complexity      376      376           
=========================================
  Files            27       27           
  Lines          3414     3414           
  Branches        171      171           
=========================================
  Hits           2690     2690           
  Misses          651      651           
  Partials         73       73           

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[shubha-rajan]

LGTM!