MONGOCRYPT-688 remove RHEL 6.2

[kevinAlbs]

Summary

Drop RHEL 6.2 builds from CI

Background & Motivation

As noted in 9b72f0c:

RHEL 6.2 builds are deprecated and may be removed in the future. The linux-x86_64-glibc_2_7-nocrypto release build may be used instead and has equivalent glibc requirements.

RHEL 6.2 builds were kept since the Java and Python drivers previously referred to RHEL 6.2 published builds (noted in MONGOCRYPT-688). The references have since been removed in Java and in Python. So this is expected not to impact drivers.

Patches for RHEL 6.2 were removed, revealing a small existing bug in the CMake make_patch_command helper that did not properly handle an empty set of arguments. This was fixed with an added check for empty arguments.