Remove 'requires_fcv_51' tag from jstests/core/double_decimal_compare.js

As of SERVER-60042, jstests/core/double_decimal_compare.js is tagged with requires_fcv_51. This is because on the implicit multiversion and all feature flags build variant, SBE is enabled for both 5.1 and 5.0, but the assertions made by the test are only true for SBE on 5.1. As such, once SBE is enabled by default on 5.1 and the implicit multiversion and all feature flags build variant no longer configures SBE on both engines, the tag can be removed from this test.