Improve/unify identifier factory references

[swissiety]

Project stored the IdentifierFactory reference via Language field and as a dedicated field.
Now only the Language is used to store it.

[codecov]

Codecov Report

Patch coverage: 63.15% and project coverage change: +0.01% 🎉

Comparison is base (889b9dd) 64.51% compared to head (5c67251) 64.53%.
Report is 4 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #648      +/-   ##
=============================================
+ Coverage      64.51%   64.53%   +0.01%     
  Complexity      3310     3310              
=============================================
  Files            311      311              
  Lines          14754    14736      -18     
  Branches        2451     2450       -1     
=============================================
- Hits            9519     9510       -9     
+ Misses          4352     4342      -10     
- Partials         883      884       +1     

Files Changed	Coverage Δ
...ootup.core/src/main/java/sootup/core/Language.java	50.00% <0.00%> (-50.00%)	⬇️
.../java/sootup/core/jimple/common/expr/JNegExpr.java	34.61% <0.00%> (ø)
.../src/main/java/sootup/core/views/AbstractView.java	50.00% <ø> (+4.54%)	⬆️
...n/java/sootup/java/core/language/JavaLanguage.java	88.88% <ø> (+11.11%)	⬆️
.../main/java/sootup/jimple/parser/JimpleProject.java	23.52% <0.00%> (+3.52%)	⬆️
...tup.core/src/main/java/sootup/core/views/View.java	66.66% <50.00%> (+44.44%)	⬆️
...sootup/java/bytecode/frontend/AsmMethodSource.java	71.40% <90.00%> (-0.05%)	⬇️
sootup.core/src/main/java/sootup/core/Project.java	81.25% <100.00%> (-2.97%)	⬇️
...a/bytecode/interceptors/EmptySwitchEliminator.java	88.88% <100.00%> (-1.12%)	⬇️
...re/src/main/java/sootup/java/core/JavaProject.java	64.86% <100.00%> (ø)

... and 3 files with indirect coverage changes

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