Remove RetryCallable from the chain when it's disabled

[igorbernstein2]

This is necessary when writing manual clients that handle their own retries: the exception wrapping is necessary, but retry callable gets in the way

[codecov-io]

Codecov Report

Merging #450 into master will decrease coverage by 0.22%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master     #450      +/-   ##
============================================
- Coverage     71.94%   71.72%   -0.23%     
  Complexity      746      746              
============================================
  Files           154      154              
  Lines          3329     3331       +2     
  Branches        256      258       +2     
============================================
- Hits           2395     2389       -6     
- Misses          837      844       +7     
- Partials         97       98       +1

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/api/gax/grpc/GrpcCallableFactory.java	69.23% <33.33%> (-2.77%)	8 <0> (+1)
.../java/com/google/api/gax/grpc/GrpcCallContext.java	71.01% <0%> (-8.7%)	21% <0%> (-1%)

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[garrettjonesgoogle]

How difficult would it be to add a test for this change?

[igorbernstein2]

I can't really think of anything simple...I can try to write a unit test that compares the relative performance of a callable with maxAttempts=0 and totalTimeout=0.00001, but I think that will be a bit too flaky

[garrettjonesgoogle]

Ok, don't worry about it then.