Many stubs for CBMC proofs are no longer necessary

[feliperodri]

Many CBMC proofs use stubs for standard C functions (e.g., memcpy) that are no longer necessary. By removing these stubs, many proofs could go from bounded to unbounded (e.g., proofs from aws_byte_buf module). This no longer makes the proofs stronger, but also could improve performance in CI. We should perform a clean up of these proofs.