visionOS: Fix the build for all BaseExtension-derived targets

https://bugs.webkit.org/show_bug.cgi?id=262414
rdar://116261778

Reviewed by Dean Jackson.

* Source/WebKit/Configurations/BaseExtension.xcconfig:
Don't specify SUPPORTED_PLATFORMS here because:

A) we get it via inheritance

B) this comes *after* we import LocalOverrides (unlike Base.xcconfig, which is *before*),
and thus overrides LocalOverrides' overrides, causing LocalOverrides to be unable to add
the visionOS platform to SUPPORTED_PLATFORMS, and causing Xcode to choose a different
platform to build for, at random (and, notably, one which it did not build the rest of WebKit for).

Canonical link: https://commits.webkit.org/268679@main

Source/WebKit/Configurations/BaseExtension.xcconfig

@@ -33,7 +33,6 @@ SWIFT_VERSION = 5.0;
 SWIFT_OPTIMIZATION_LEVEL = -O;
 SWIFT_OPTIMIZATION_LEVEL[config=Debug] = -Onone;
 PRODUCT_NAME = $(TARGET_NAME);
-SUPPORTED_PLATFORMS = appletvos appletvsimulator iphoneos iphonesimulator macosx watchos watchsimulator;
 SUPPORTS_MACCATALYST = NO;
 SUPPORTS_MAC_DESIGNED_FOR_IPHONE_IPAD = NO;
 TARGETED_DEVICE_FAMILY = 1,2;