Fix: Remove singleton capabilities

[localheinz]

This PR

• removes FixtureFactory::getAsSingleton(), FixtureFactory::setSingleton(), and FixtureFactory::unsetSingleton()

Follows #1.

💁‍♂ I have never used it, not sure if I will miss it.

[codecov]

Codecov Report

Merging #124 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #124      +/-   ##
============================================
+ Coverage     94.23%   94.32%   +0.09%     
+ Complexity       74       67       -7     
============================================
  Files            10       10              
  Lines           208      194      -14     
============================================
- Hits            196      183      -13     
+ Misses           12       11       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/FieldDefinition.php	100.00% <ø> (ø)	6.00 <0.00> (ø)
src/FixtureFactory.php	91.66% <ø> (-0.14%)	38.00 <0.00> (-7.00)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b503b60...8096f34. Read the comment docs.