Remove unused functions.

This CL is essentially reverting following changes:
- "Serialize and deserialize supported axes."
  I4086fb887e13f872390b533584bce6f1d5598ea0
- "Make SparseBitSet serializable."
  I0463138adcf234739bb3ce1cdadf382021921f3e

Bug: 36232655
Test: N/A
Change-Id: I25c701e1805e92b01034142147a9925f86533819

• include/minikin/FontFamily.h[diff]
• include/minikin/SparseBitSet.h[diff]
• libs/minikin/FontFamily.cpp[diff]
• libs/minikin/SparseBitSet.cpp[diff]
• tests/perftests/FontFamily.cpp[diff]
• tests/unittest/FontFamilyTest.cpp[diff]
• tests/unittest/SparseBitSetTest.cpp[diff]