BuildMacOSUniversalBinary: Remove architecture-specific macOS deployment target flags

I can't see any situation in the foreseeable future where we need to target different macOS versions for each architecture.

BuildMacOSUniversalBinary.py

@@ -60,10 +60,6 @@ DEFAULT_CONFIG = {
     # permissions needed for ARM builds
     "codesign_identity":  "-",
 
-    # Minimum macOS version for each architecture slice
-    "arm64_mac_os_deployment_target":  "11.0.0",
-    "x86_64_mac_os_deployment_target": "10.15.0",
-
     # CMake Generator to use for building
     "generator": "Unix Makefiles",
     "build_type": "Release",
@@ -146,11 +142,6 @@ def parse_args(conf=DEFAULT_CONFIG):
              help=f"Install path for {arch} qt5 libraries",
              default=conf[arch+"_qt5_path"])
 
-        parser.add_argument(
-             f"--{arch}_mac_os_deployment_target",
-             help=f"Deployment architecture for {arch} slice",
-             default=conf[arch+"_mac_os_deployment_target"])
-
     return vars(parser.parse_args())
 
 
@@ -297,8 +288,6 @@ def build(config):
                 "-DCMAKE_PREFIX_PATH="+prefix_path,
                 "-DCMAKE_SYSTEM_PROCESSOR="+arch,
                 "-DCMAKE_IGNORE_PATH="+ignore_path,
-                "-DCMAKE_OSX_DEPLOYMENT_TARGET="
-                + config[arch+"_mac_os_deployment_target"],
                 "-DMACOS_CODE_SIGNING_IDENTITY="
                 + config["codesign_identity"],
                 '-DMACOS_CODE_SIGNING="ON"',