Remove the rule on identifier syntax

[dariavladykina]

The style guide currently lists the following guidelines on identifiers:

The current guidance is: "In identifiers, only use lowercase basic Latin alphabetic and numeric characters and - (hyphen).
Do not use _ and . characters, as they may hurt search engine optimization."

The style checker still considers the period ( . ) to be the correct prefix for identifiers.

Example output: Identifier of book-gnome-user has an unusual prefix. Use the prefix book. instead.
Same for part., cha., sec., fig., etc.

The issues reported occupy about 40-50% of the entire output.
This rule should be removed.