Support for building with macOS SDK < 10.12 is long gone

...according to 372d2d78 "Update OSX SDK
checking".  (With the current code, there was still a possibility to pass in an
explicit --with-macosx-sdk= with a value of 10.9, 10.10, or 10.11, but I assume
that was an oversight rather than a desirable feature, and that nobody, esp. not
the "official" TDF builds, actually used it.)

Change-Id: Ib3cb7309f6aa7c506687e9746acfcf365422901f
Reviewed-on: https://gerrit.libreoffice.org/60375
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -2742,15 +2742,6 @@ if test $_os = Darwin -o $_os = iOS; then
 
 
     case $with_macosx_sdk in
-    10.9)
-        MACOSX_SDK_VERSION=1090
-        ;;
-    10.10)
-        MACOSX_SDK_VERSION=101000
-        ;;
-    10.11)
-        MACOSX_SDK_VERSION=101100
-        ;;
     10.12)
         MACOSX_SDK_VERSION=101200
         ;;