refactor: remove DB generic

[bsbds]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  The DB generic is redundant as no other type will ever implement StorageApi in Xline. This change aims to enhance readability of the codebase.

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.32479% with 16 lines in your changes missing coverage. Please review.

Project coverage is 75.70%. Comparing base (e35b35a) to head (d836804). Report is 117 commits behind head on master.

Files	Patch %	Lines
crates/xline/src/server/command.rs	64.70%	3 Missing and 3 partials :warning:
crates/xline/src/server/maintenance.rs	62.50%	1 Missing and 5 partials :warning:
crates/xline/src/server/xline_server.rs	89.65%	0 Missing and 3 partials :warning:
crates/xline/src/storage/db.rs	94.73%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #806 +/- ## ========================================== + Coverage 75.55% 75.70% +0.15% ========================================== Files 180 187 +7 Lines 26938 27822 +884 Branches 26938 27822 +884 ========================================== + Hits 20353 21063 +710 - Misses 5366 5470 +104 - Partials 1219 1289 +70 ```

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[kamyuentse]

Though xline currently has only one StorageApi impl，the generic is make sense for the community if someone want to use another storage engine.

[bsbds]

Though xline currently has only one StorageApi impl，the generic is make sense for the community if someone want to use another storage engine.

The engine crate already provides a level of abstraction, I think introducing the StorageApi would be redundant.

[bsbds]

If we remove the DB generic here, is it necessary to keep StorageApi trait?

removed