[indic-generator] Remove three unneeded Myanmar overrides U+AA74-6 These three characters have Indic_Syllabic_Category=Consonant_Placeholder. The original evidence that prompted these overrides says they can take tone marks. They are not subjoined: Khamti Shan apparently does not use subjoined characters at all. Therefore, PLACEHOLDER is good enough and these need not be overridden to C. https://www.unicode.org/L2/L2008/08276-khamti-proposal.pdf https://github.com/harfbuzz/harfbuzz/pull/3648#discussion_r894640713