Editorial: remove some "are both" usages

[michaelficarra]

This doesn't remove all occurrences, but the other occurrences are either in prose/asserts or we talked about them and they are justified because they form a nice pattern of spec steps. Typically, we avoid using this structure.

Additionally, this particular test is done in 6 places, 4 of which are already using our preferred phrasing.

[syg]

Do we typically avoid using this?

[michaelficarra]

@syg Yes. https://github.com/tc39/ecma262/pull/2877

• prefer "if a is c and b is c" over "if a and b are both c"