build: drop extra qbconfig.h rule for auto_check_header self-test

Although this is not as rigorously correct as the previous arrangement
(introduced with 80834ea), i.e., from the perspective of
$(abs_top_builddir)/tests/Makefile (generated from from
$(top_srcdir)/tests/Makefile.am), for which $(top_builddir)
is literally "..":

• qbconfig.h is generated from $(top_srcdir)/include/qb/qbconfig.h.in into $(top_builddir)/include/qb directory, and consumed from here

• rest of these header files are consumed directly from $(top_srcdir)/include/qb directory

this new simplified variant:

• avoids code duplication (DRY)

• works thanks to VPATH in $(abs_top_builddir)/tests/Makefile pointing to $(top_srcdir)/tests, which means that unsatisfied dependencies on "$(top_builddir)/include/qb/%.h" (resolved to "../include/qb/%.h") will be subsequently sought as "$(top_srcdir)/tests/../include/qb/%.h", which has the same effect as previous explicit "$(top_srcdir)/include/qb/%.h" specification (note that such automatic VPATH provision is supported by automake "since beginning" so no backward incompatibility is expected)