Static error function cannot be removed from CBMC report

Static error function `aws_raise_error` cannot be removed from CBMC reports even though it comes from project sources. In particular, it comes from an include of error.c from the AWS-C-Common project.

This issue could be related to #624 , although in this case the proof result is not affected.

Here is the list of proofs related to the issue:
* `aws_cryptosdk_edk_list_clean_up`
* `aws_cryptosdk_edk_list_clear`
* `aws_cryptosdk_enc_materials_destroy`
* `aws_cryptosdk_keyring_trace_clean_up`
* `aws_cryptosdk_keyring_trace_clear`
* `aws_cryptosdk_keyring_trace_eq`

Start by reproducing the CBMC reports for the six listed proofs and trace how the included AWS-C-Common error.c contributes aws_raise_error. Compare the report behavior with issue #624; done means the static error function is removed from these reports without changing the proof results.

Written by the indexing model from the issue text.