geos: revert added legacysupport and blacklist macOS 10.15 provided Xcode clang

Reverts commit 504569d9f0

See: https://trac.macports.org/ticket/73172

science/geos/Portfile

@@ -2,7 +2,6 @@
 
 PortSystem          1.0
 PortGroup           cmake 1.1
-PortGroup           legacysupport 1.1
 
 # NOTE: When updating this port, check whether the dylib name and/or version
 # changes. If so, all dependents will need to be rev-bumped.
@@ -37,10 +36,8 @@ compiler.cxx_standard 2017
 # and linking errors observed for Xcode 5.1.1 (clang 503) on 10.8.
 # See https://github.com/macports/macports-ports/pull/6159
 # Also avoid compilation errors observed for Xcode 6, 7, and 8.
-compiler.blacklist-append {clang < 900}
-
-legacysupport.newest_darwin_requires_legacy \
-                    19
+# See also https://trac.macports.org/ticket/73172
+compiler.blacklist-append {clang < 1205}
 
 use_parallel_build  yes