Remove old special cases for disabling LTO

Right now, LTO is disabled for RPM builds in general. The block removed by this
commit is no longer in use (the lto_build condition has no effect at the
moment). Therefore remove it to be consistent with the master branch.

icinga2.spec

@@ -68,14 +68,6 @@
 %bcond_without unity_build
 %endif
 
-# Disable lto_build for older SUSE versions
-# see https://github.com/Icinga/icinga2/issues/7149
-%if "%{_vendor}" == "suse" && 0%{?suse_version} <= 1315
-%bcond_with lto_build
-%else # suse_version 1315
-%bcond_without lto_build
-%endif # suse_version 1315
-
 %bcond_with systemd_and_init
 %bcond_without compat
 %bcond_without livestatus