Undo "check for boost/process/child.hpp in configure.ac if --enable-gtk3-kde5"

This reverts commit c7a13205.
It's no longer required since we ported away from boost.:process

Change-Id: I53c5b994c3ba2801823ba0d6ab25f43223db1476
Reviewed-on: https://gerrit.libreoffice.org/48491Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

configure.ac

@@ -8716,10 +8716,6 @@ if test "$with_system_boost" = "yes"; then
        [AC_MSG_ERROR(boost/shared_ptr.hpp not found. install boost)], [])
     AC_CHECK_HEADER(boost/spirit/include/classic_core.hpp, [],
        [AC_MSG_ERROR(boost/spirit/include/classic_core.hpp not found. install boost >= 1.36)], [])
-if test "x$enable_gtk3_kde5" = "xyes"; then
-    AC_CHECK_HEADER(boost/process/child.hpp, [],
-       [AC_MSG_ERROR(boost/process/child.hpp not found. install boost >= 1.64)], [])
-fi
     CXXFLAGS=$save_CXXFLAGS
     AC_LANG_POP([C++])
     # this is in m4/ax_boost_base.m4