Drop abandoned experimental features and experimental pragmas

[cameel]

We have a few experimental features that are no longer being developed and became essentially technical debt. We also have some old experimental pragmas that no longer have a purpose. We should remove them all.

No deprecation needed since it was all experimental.

Features to remove:

• LSP
  • The development effectively ceased before it became stable and there are no plans to revive it.
• Generic Solidity prototype (pragma experimental solidity)
  • The development moved to solcore. When the new prototype is finished, it will be reimplemented from scratch.

Pragmas to remove:

• pragma experimental ABIEncoderV2
  • Replaced by pragma abicoder in 0.7.5. It was scheduled for deprecation in 0.8.0, but since it's experimental deprecation was not actually necessary.

We will treat pragma experimental SMTChecker as an exception and wait with removal for 0.9.0 (#15874). It's technically experimental, but the feature is prominent enough that we can give it a deprecation period.