Remove v8_enable_shared_ro_heap=false and v8_enable_verify_heap=false

[goffrie]

v8_enable_shared_ro_heap is no longer configurable since https://github.com/v8/v8/commit/b3054f77d65ea96ddf6e0a526abadebd91d7114e.

This also removes the justification for setting v8_enable_verify_heap=false so remove that too.