Remove unused multi-version iOS variables from WebKitTargetConditionals.xcconfig.

They aren't used anywhere inside WebKit, and they haven't been kept up-to-date after `WK_IOS_17` was added, so they likely aren't used any longer.

Referring to these Xcode variables in Configurations/WebKitTargetConditionals.xcconfig:

'''
WK_IOS_13_14 = $(WK_IOS_13_14_$(WK_IOS_BEFORE_13));
WK_IOS_13_14_ = $(WK_IOS_14);
WK_IOS_13_14_YES = _IOS_BEFORE_13;

WK_IOS_13_15 = $(WK_IOS_13_15_$(WK_IOS_BEFORE_13));
WK_IOS_13_15_ = $(WK_IOS_15);
WK_IOS_13_15_YES = _IOS_BEFORE_13;

WK_IOS_13_16 = $(WK_IOS_13_16_$(WK_IOS_BEFORE_13));
WK_IOS_13_16_ = $(WK_IOS_16);
WK_IOS_13_16_YES = _IOS_BEFORE_13;

[...]

WK_IOS_14_15 = $(WK_IOS_14_15_$(WK_IOS_BEFORE_14));
WK_IOS_14_15_ = $(WK_IOS_15);
WK_IOS_14_15_YES = _IOS_BEFORE_14;

WK_IOS_14_16 = $(WK_IOS_14_16_$(WK_IOS_BEFORE_14));
WK_IOS_14_16_ = $(WK_IOS_16);
WK_IOS_14_16_YES = _IOS_BEFORE_14;

[...]

WK_IOS_15_16 = $(WK_IOS_15_16_$(WK_IOS_BEFORE_15));
WK_IOS_15_16_ = $(WK_IOS_16);
WK_IOS_15_16_YES = _IOS_BEFORE_15;

[...]

WK_IOS_16_17 = $(WK_IOS_16_17_$(WK_IOS_BEFORE_16));
WK_IOS_16_17_ = $(WK_IOS_17);
WK_IOS_16_17_YES = _IOS_BEFORE_16;
'''