`process_withdraw` duplicates `check_mint` check.

[enriquefynn]

Speaking with kkonevets, thanks for the report 👍, he found out that we check twice for the mint at:

solido/anker/src/processor.rs

Line 410 in 0d9a19c

anker.check_mint(accounts.b_sol_mint)?;

It doesn't impact our deployment 😅 but it does consume a bit more computing, or maybe the compiler is smart enough to remove/optimize this.
Anyways we should remove the duplicate check.

[ruuda]

Good find!