CSUB-699: Remove switch_to_pos() extrinsic b/c mainnet has now been migrated to PoS

[atodorov]

• keep storage item b/c it is important
• remove related tests and test jobs

Description of proposed changes

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Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[codecov-commenter]

Codecov Report

Merging #1289 (885322b) into dev (38ee975) will increase coverage by 62.36%.
Report is 11 commits behind head on dev.
The diff coverage is n/a.

❗ Current head 885322b differs from pull request most recent head c93d72f. Consider uploading reports for the commit c93d72f to get more accurate results

@@            Coverage Diff             @@
##             dev    #1289       +/-   ##
==========================================
+ Coverage   8.32%   70.69%   +62.36%     
==========================================
  Files         28      103       +75     
  Lines        889    12082    +11193     
  Branches     114      114               
==========================================
+ Hits          74     8541     +8467     
- Misses       815     3532     +2717     
- Partials       0        9        +9     

Files Changed	Coverage Δ
pallets/pos-switch/src/lib.rs	71.42% <ø> (ø)
runtime/src/lib.rs	17.34% <ø> (ø)
runtime/src/version.rs	0.00% <ø> (ø)

... and 80 files with indirect coverage changes

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[zacharyfrederick]

The changes look good. Should we communicate this removal to the community? Seems prudent from a transparency perspective

Edit: Maybe a better question for marketing

[github-actions]

For full LLVM coverage report click here!

[pLabarta]

LGTM, later on I would consider options for removing the whole pallet while migrating SwitchBlockNumber somewhere else