delete SKIP_TOKENS_PARSER setting

[noviluni]

As mentioned here: #722

I think we can delete this setting based on:

• It has been never documented
• There isn't any known use case where a user needs to change it.
• There are 0 references in issues, etc: https://github.com/scrapinghub/dateparser/search?q=SKIP_TOKENS_PARSER (apart from the mentioned related PR)

[codecov]

Codecov Report

Merging #728 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #728      +/-   ##
==========================================
+ Coverage   95.23%   95.35%   +0.12%     
==========================================
  Files         302      304       +2     
  Lines        2560     2585      +25     
==========================================
+ Hits         2438     2465      +27     
+ Misses        122      120       -2     

Impacted Files	Coverage Δ
dateparser/parser.py	98.92% <100.00%> (-0.01%)	⬇️
dateparser/calendars/__init__.py	95.06% <0.00%> (-0.45%)	⬇️
dateparser/languages/validation.py	93.43% <0.00%> (-0.44%)	⬇️
dateparser/search/detection.py	94.23% <0.00%> (-0.11%)	⬇️
dateparser/date.py	98.00% <0.00%> (-0.01%)	⬇️
dateparser/search/search.py	99.35% <0.00%> (-0.01%)	⬇️
dateparser/calendars/jalali_parser.py	100.00% <0.00%> (ø)
dateparser/__init__.py	100.00% <0.00%> (ø)
dateparser/conf.py	100.00% <0.00%> (ø)
... and 1 more

---

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 6dc38bb...bf58427. Read the comment docs.

[codecov]

Codecov Report

Merging #728 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files         304      304           
  Lines        2662     2663    +1     
=======================================
+ Hits         2542     2543    +1     
  Misses        120      120           

Impacted Files	Coverage Δ
dateparser/parser.py	99.02% <100.00%> (+<0.01%)	⬆️

---

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 b256f14...26db985. Read the comment docs.