The dependency is redundant, because the library is now bundled into
tarantool (see PR #6877).

There is a practical reason to remove it: this way we'll verify that
presence of system libunwind headers is not necessary to build
tarantool. In other words, we'll verify that #8025 will not appear
again.

Follows up #8025
Follows up #6877
Part of #6998

NO_DOC=No user visible changes, just some validation for a build fix.
NO_TEST=See NO_DOC.
NO_CHANGELOG=See a changelog entry in the previous commit.