boost-regex: Remove superfluous blueprint of this header-only library
header-only library -> we don't have to build anything; don't depend on bjam
We don't have to install anything because boost-headers already includes everything.
I have tested this by building the stellarsolver package which depends on boost-regex. Apparently, it needs boost-regex only for the Windows build. cmake didn't find boost-regex,
-- Could NOT find Boost (missing: regex) (found suitable version "1.84.0", minimum required is "1.45.0")
but the build still succeeded: https://invent.kde.org/packaging/craft-blueprints-kde/-/jobs/2047711