Now that all 'const' declarations are of the ES2015 variety, the only
use of CONST_LEGACY is for function name bindings in sloppy mode
named function expressions.

This patch aims to delete all code meant to handle other cases, which
mostly had to do with hole initialization/hole checks. Since function
name bindings are initialized at entry to a function, it's impossible
to ever observe one in an uninitialized state.

To simplify the patch further, it removes the `IMPORT` VariableMode,
as it's not likely to be needed (IMPORT is identical to CONST for
the purpose of VariableMode).

Review URL: https://codereview.chromium.org/1895973002

Cr-Commit-Position: refs/heads/master@{#35632}