[fidlc][c] Use resolved enum values from library

Given previous changes, enum member values are now validated and
resolved at compile time and should no longer need to be parsed by the
C backend. This change removes the parsing from the C backend and
instead, reads the resolved values.

FIDL-304 #comment

Test: No regressions on C bindings.
Change-Id: If9e97afc97da8b1c3f66e5cd337d2b4c3ef37c94