Remove xtokens MigrationPhase

[zjb0807]

Add migrations to clean the storage.

parameter_types! {
    pub const XTokensPallet: &'static str = "XTokens";
    pub const MigrationStatus: &'static str = "MigrationStatus";
}

pub type Migrations = (
    RemoveStorage<XTokensPallet, MigrationStatus, RocksDbWeight>,
);

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.65%. Comparing base (937ac46) to head (e35eba3).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
xtokens/src/lib.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1035      +/-   ##
==========================================
- Coverage   65.97%   65.65%   -0.32%     
==========================================
  Files          48       48              
  Lines        3697     3689       -8     
==========================================
- Hits         2439     2422      -17     
- Misses       1258     1267       +9     

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