cleanup: Remove unneeded build/Dockerfiles and cleanup build.sh

[ghost]

Context:
#2802 (comment)
for rationale.

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This change is 

[coryan]

This looks good but I need to merge some changes internally to disable the centos-7 build first, and then I can run the Kokoro builds.

[codecov]

Codecov Report

Merging #2926 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2926     +/-   ##
=========================================
- Coverage   90.75%   90.66%   -0.1%     
=========================================
  Files         297      297             
  Lines       19943    20021     +78     
=========================================
+ Hits        18100    18152     +52     
- Misses       1843     1869     +26

Impacted Files	Coverage Δ
google/cloud/storage/oauth2/credentials.h	33.33% <0%> (-33.34%)	⬇️
google/cloud/storage/internal/complex_option.h	66.66% <0%> (-16.67%)	⬇️
google/cloud/storage/well_known_parameters.h	68.88% <0%> (-15.56%)	⬇️
google/cloud/storage/object_stream.h	98.14% <0%> (-1.86%)	⬇️
google/cloud/storage/internal/curl_client.cc	94.7% <0%> (-0.34%)	⬇️
google/cloud/storage/internal/object_streambuf.h	100% <0%> (ø)	⬆️
google/cloud/bigtable/mutations.h	100% <0%> (ø)	⬆️
google/cloud/storage/object_metadata.h	100% <0%> (ø)	⬆️
google/cloud/internal/retry_policy.h	100% <0%> (ø)	⬆️
google/cloud/bigtable/examples/data_snippets.cc	85.16% <0%> (ø)	⬆️
... and 4 more

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