Don't treat @ParametricNullness as @Nullable in JSpecify mode

[msridhar]

Fixes #1173

In JSpecify mode, we should rely on our checking of generic types and not treat @ParametricNullness as a @Nullable annotation.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.22%. Comparing base (3da2c82) to head (983051f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...away/src/main/java/com/uber/nullaway/Nullness.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1174      +/-   ##
============================================
+ Coverage     88.19%   88.22%   +0.02%     
- Complexity     2310     2311       +1     
============================================
  Files            88       88              
  Lines          7539     7541       +2     
  Branches       1508     1510       +2     
============================================
+ Hits           6649     6653       +4     
+ Misses          446      445       -1     
+ Partials        444      443       -1     

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