coding guidelines: remove bool advice that conflicts with clang-tidy

[Habbie]

Short description

noted by @BozhanL in #14993

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 12633751652

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 93 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.04%) to 64.819%

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Files with Coverage Reduction	New Missed Lines	%
pdns/backends/gsql/gsqlbackend.hh	1	97.71%
pdns/recursordist/lwres.cc	2	69.66%
ext/json11/json11.cpp	2	62.72%
pdns/sstuff.hh	2	56.83%
pdns/opensslsigners.cc	2	61.13%
pdns/stubresolver.cc	3	77.58%
pdns/misc.cc	3	64.45%
pdns/dnsdistdist/dnsdist-carbon.cc	6	61.6%
pdns/recursordist/test-syncres_cc1.cc	7	89.68%
pdns/recursordist/syncres.cc	10	80.09%

Totals
Change from base Build 12632986338:	-0.04%
Covered Lines:	126124
Relevant Lines:	163796

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💛 - Coveralls