Remove warnOnLegacyHierarchySeparator property

[yannbf]

This was already removed in Storybook 7, but its types weren't:

storybook/code/lib/core-server/src/presets/common-preset.ts

Line 190 in 94f4361

warnOnLegacyHierarchySeparator: true,

storybook/code/lib/types/src/modules/core-common.ts

Line 382 in 3b94da1

warnOnLegacyHierarchySeparator?: boolean;

storybook/code/lib/core-server/src/utils/getStoryIndexGenerator.ts

Line 13 in ff88e04

warnOnLegacyHierarchySeparator?: boolean;