This is better reviewed commit by commit.
This is a massive PR about removing legacy code, might split it in multiple PRs, but this is actually the first stable state in which I was able to make everything compile and work

What is done and what is missing:

• 
 Remove all sharedHistory code
• 
 Remove MX_CRYPTO macros
• 
 MXUserTrustLevel can be removed and replaced with a single isVerified boolean, given that we no longer need to distinguish local and crossSigning trust status
• 
 NSCoding implementations from MXCrossSigningInfo and others can be removed, as they are no longer persisted on the iOS side, but rather rust side
• 
 Remove a number of unnecessary options from MXSDKOptions, such as enableRoomSharedHistoryOnInvite or enableSymmetricBackup
• 
 A large number of integration tests access a legacyCrypto or legacyCrossSigning property which should be removed. These tests then have to be either refactored or removed entirely
• 
 Remove MXBackgroundCryptoStore
• 
 Legacy MXCryptoStore and MXRealmCryptoStore need to remain in place for the purpose of data migration to rust crypto, but only a few methods are required. (NOTE: I actually removed them since we don’t want to allow migration anymore)
• 
 Remove MXSecretShareManager, and the requestPrivateKeys in MXKeyBackup which is no longer used
• 
 This will enable the removal of many other associated classes, such as MXLegacySasTransaction, MXMegolmEncryption, MXMegolmDecryption etc
• Remove main ...Legacy... classes such as MXLegacyCrypto, MXLegacyCrossSigning, MXLegacyKeyVerificationManager etc