m4: Drop PKG_PROG_PKG_CONFIG compatibility code

This was needed for RHEL 4 vintage distributions, which we
haven't supported for a long time now.

m4/virt-pkgconfig-back-compat.m4

-dnl
-dnl To support the old pkg-config from RHEL4 vintage, we need
-dnl to define the PKG_PROG_PKG_CONFIG macro if its not already
-dnl present
-m4_ifndef([PKG_PROG_PKG_CONFIG],
-  [AC_DEFUN([PKG_PROG_PKG_CONFIG],
-[m4_pattern_forbid([^_?PKG_[A-Z_]+$])
-m4_pattern_allow([^PKG_CONFIG(_PATH)?$])
-AC_ARG_VAR([PKG_CONFIG], [path to pkg-config utility])dnl
-if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
-        AC_PATH_TOOL([PKG_CONFIG], [pkg-config])
-fi
-if test -n "$PKG_CONFIG"; then
-        _pkg_min_version=m4_default([$1], [0.9.0])
-        AC_MSG_CHECKING([pkg-config is at least version $_pkg_min_version])
-        if $PKG_CONFIG --atleast-pkgconfig-version $_pkg_min_version; then
-                AC_MSG_RESULT([yes])
-        else
-                AC_MSG_RESULT([no])
-                PKG_CONFIG=""
-        fi
-fi[]dnl
-])])