[config] Remove duplicated entry from the checker severity map
`misc-non-private-member-variables-in-classes` checker is defined multiple times
in the `checker_severity_map.json` file. This commit will keep only one.
"misc-move-constructor-init": "MEDIUM",
"misc-move-forwarding-reference": "MEDIUM",
"misc-multiple-statement-macro": "MEDIUM",
- "misc-non-private-member-variables-in-classes": "LOW",
"misc-new-delete-overloads": "MEDIUM",
"misc-noexcept-move-constructor": "MEDIUM",
"misc-non-copyable-objects": "HIGH",
