refactor(vtk): remove deprecated export_* functions

[wpbonelli]

• deprecated in 3.3.5 (March 2022)
• update docstrings which previously referred to these methods, refer now to Vtk class

[codecov]

Codecov Report

Merging #1890 (966f26c) into develop (70b9a37) will increase coverage by 0.1%.
Report is 5 commits behind head on develop.
The diff coverage is n/a.

❗ Current head 966f26c differs from pull request most recent head f3cf448. Consider uploading reports for the commit f3cf448 to get more accurate results

@@            Coverage Diff            @@
##           develop   #1890     +/-   ##
=========================================
+ Coverage     72.2%   72.4%   +0.1%     
=========================================
  Files          257     257             
  Lines        56488   56373    -115     
=========================================
- Hits         40836   40829      -7     
+ Misses       15652   15544    -108     

Files Changed	Coverage Δ
flopy/export/utils.py	64.7% <ø> (-0.1%)	⬇️
flopy/export/vtk.py	79.8% <ø> (+9.5%)	⬆️
flopy/mf6/mfmodel.py	58.7% <ø> (ø)

[langevin-usgs]

should "Vtk ctor" be "Vtk actor"?

[wpbonelli]

thanks @langevin-usgs updated to initializer, as ctor (constructor) was improper use per python conventions anyway