patch: delete legacy code

[SeekingMeaning]

SoftwareSpec#add_legacy_patches and Patch#normalize_legacy_patches were added almost 6 years ago; let's delete them

See commit 665b14c

$ grep -r 'normalize_legacy_patches' Library/Homebrew
./software_spec.rb:    list = Patch.normalize_legacy_patches(list)
./patch.rb:  def self.normalize_legacy_patches(list)

$ grep -r 'add_legacy_patches' Library/Homebrew
./software_spec.rb:  def add_legacy_patches(list)

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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[BrewTestBot]

Review period will end on 2021-01-15 at 02:44:38 UTC.

[MikeMcQuaid]

Thanks! Always nice to 🔥 💀 code.

[BrewTestBot]

Review period ended.