feat(mainnet): continue preparing mainnet runtime.

[peterwht]

The commit names and content will explain the changes in this PR:
refactor(mainnet): remove pop api chain extension
refactor(mainnet): change live to mainnet runtime` -> the name "mainnet" name is consistent with "devnet" and "testnet"
chore(mainnet): remove unnecessary migrations
chore(mainnet): secure contracts config
feat(mainnet): add pop-up config, and add mainnet option in command.rs
feat(CI): add mainnet to CI release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 7.44%. Comparing base (c7e2026) to head (33322cb).

Files with missing lines	Patch %	Lines
runtime/mainnet/src/config/proxy.rs	0.00%	15 Missing ⚠️
node/src/chain_spec.rs	0.00%	9 Missing ⚠️
node/src/command.rs	0.00%	6 Missing ⚠️

@@                      Coverage Diff                      @@
##           al3mart/chore-registration    #239      +/-   ##
=============================================================
+ Coverage                        7.11%   7.44%   +0.32%     
=============================================================
  Files                              36      34       -2     
  Lines                            2698    2458     -240     
  Branches                         2698    2458     -240     
=============================================================
- Hits                              192     183       -9     
+ Misses                           2506    2275     -231     

Files with missing lines	Coverage Δ
runtime/mainnet/src/config/xcm.rs	0.00% <ø> (ø)
runtime/mainnet/src/lib.rs	5.47% <ø> (ø)
runtime/mainnet/src/weights/block_weights.rs	81.81% <ø> (ø)
runtime/mainnet/src/weights/extrinsic_weights.rs	81.81% <ø> (ø)
runtime/mainnet/src/weights/paritydb_weights.rs	73.33% <ø> (ø)
runtime/mainnet/src/weights/rocksdb_weights.rs	73.33% <ø> (ø)
node/src/command.rs	0.00% <0.00%> (ø)
node/src/chain_spec.rs	0.00% <0.00%> (ø)
runtime/mainnet/src/config/proxy.rs	0.00% <0.00%> (ø)

[al3mart]

Awesome!

I've created a PR to your branch removing assets & contracts: #241
Let's get this change merge here.