Remove unused flag DEXOPT_ONLY_SHARED_DEX.

It's not set anywhere except in tests. History shows it was implemented
in ce543973 and partially reverted in 1ba0b8ac. However a flag
refactoring in 1d0e83d2 got in between, so some parts of the flag
handling code wasn't cleaned up in the revert.

The flag is removed keep parity with ART Services, where it is not
(currently) planned to be supported.

Test: atest DexoptOptionsTests
Bug: 251903639
Change-Id: I09b3eb09636c2add529e1c77e938e073d634fe50