Remove deprecated L2Projector methods

[fredrikekre]

This old L2 projection API have been deprecated for quite some releasese now and it is time to clean up.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04 ⚠️

Comparison is base (ccd70b5) 92.73% compared to head (397dbc9) 92.70%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
- Coverage   92.73%   92.70%   -0.04%     
==========================================
  Files          30       30              
  Lines        4406     4387      -19     
==========================================
- Hits         4086     4067      -19     
  Misses        320      320              

Impacted Files	Coverage Δ
src/L2_projection.jl	100.00% <100.00%> (ø)

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