package/cc-tool: drop BR2_PACKAGE_BOOST_SIGNALS select

package/cc-tool: drop BR2_PACKAGE_BOOST_SIGNALS select

cc-tool depends on signals2 not signals, indeed only signals2 is used
in src/data/progress_watcher.h and BOOST_SIGNALS defined in
m4/boost.m4 is never used in configure.ac.

There is no need to select any sort of BR2_PACKAGE_BOOST_SIGNALS2
option, as signals2 is a header-only boost library, and such
header-only boost libraries do not have any Config.in options, as they
are all always installed with the base boost library.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: tweak commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>