config: Make CLEAR with a nonempty value into a nonfatal assertion.

[nmathewson]

When we parse a CLEAR line (e.g., "/OrPort" or /OrPort blah blah"),
we always suppress the value, even if one exists. That means that
the block of code was meant to handle CLEAR lines didn't actually do
anything, since we previously handled them the same way as with
other empty values.

Closes ticket 31529.

[coveralls]

Pull Request Test Coverage Report for Build 6053

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 63.055%

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Totals
Change from base Build 6051:	0.0%
Covered Lines:	47620
Relevant Lines:	75521

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💛 - Coveralls