Delete method kept for layoutlib

Typeface.buildSystemFallback was kept only because it was used in
layoutlib. Change I33f27f62 (commit a65f963) removed that last use case,
so this now deletes the unused method.

Bug: 112196940
Test: atest FrameworksCoreTests:TypefaceTest
Change-Id: I19b793d34c1ce16d5dc7430c08746261cd52b4b2