cleanup: remove remnant IAM deprecation macros

[devbww]

GOOGLE_CLOUD_CPP_BIGTABLE_IAM_DEPRECATED disappeared in #8652.
GOOGLE_CLOUD_CPP_IAM_DEPRECATED disappeared in #8667.

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This change is 

[google-cloud-cpp-bot]

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[dbolduc]

Oops, I missed this in #8667. Thanks for noticing and cleaning up!

[dbolduc]

Hmm.. I recently added

google-cloud-cpp/google/cloud/bigtable/version.h

Lines 22 to 26 in b09c2f3

	#define GOOGLE_CLOUD_CPP_BIGTABLE_DATA_CLIENT_DEPRECATED(name) \
	GOOGLE_CLOUD_CPP_DEPRECATED( \
	"google::cloud::bigtable::DataClient::" name \
	" is deprecated, and will be removed on or shortly after 2023-05-01." \
	" See GitHub issue #8800 for more information.")

and the doxygen seems fine (although I don't remember what the original problem was). Maybe it is fine because it reduces to GOOGLE_CLOUD_CPP_DEPRECATED? If so, then maybe we can remove the spanner macro from this list.

[devbww]

My understanding is that it should be a problem if doxygen saw any code that used GOOGLE_CLOUD_CPP_BIGTABLE_DATA_CLIENT_DEPRECATED. So you may want to investigate.

[The original problem is that doxygen doesn't usually expand macros during its parse, so those attribute macros look like syntax errors.]

[dbolduc]

Ack. I think the GOOGLE_CLOUD_CPP_BIGTABLE_DATA_CLIENT_DEPRECATED macro is only used in tests and internal classes. So we should be fine.

[codecov]

Codecov Report

Merging #8873 (4f11776) into main (b33dcb7) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8873   +/-   ##
=======================================
  Coverage   93.20%   93.20%           
=======================================
  Files        1474     1474           
  Lines      124487   124487           
=======================================
+ Hits       116033   116034    +1     
+ Misses       8454     8453    -1     

Impacted Files	Coverage Δ
google/cloud/version.h	100.00% <ø> (ø)
google/cloud/completion_queue_test.cc	97.13% <0.00%> (+0.19%)	⬆️

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