Remove unused WhichFormula ENDPOINT and DATABASE_FILE constants

`brew which-formula` is a `ShellCommand` implemented in Bash
(utils/executables.sh), so these Ruby constants were never read at runtime;
only the spec referenced `DATABASE_FILE`. Remove both and have the spec write
the database to the same path `Homebrew::API.write_executables_file!` uses.

Claude-Session: https://claude.ai/code/session_011BgWRBydtdkvt8ocqH4nj9

Library/Homebrew/cmd/which-formula.rb

@@ -11,9 +11,6 @@ require "shell_command"
 module Homebrew
   module Cmd
     class WhichFormula < AbstractCommand
-      ENDPOINT = "internal/executables.txt"
-      DATABASE_FILE = T.let((Homebrew::API::HOMEBREW_CACHE_API/ENDPOINT).freeze, Pathname)
-
       include ShellCommand
 
       cmd_args do

Library/Homebrew/test/cmd/which-formula_spec.rb

@@ -19,11 +19,9 @@ RSpec.describe Homebrew::Cmd::WhichFormula do
     end
 
     before do
-      # Override DATABASE_FILE to use test environment's HOMEBREW_CACHE
-      test_db_file = HOMEBREW_CACHE/"api"/Homebrew::Cmd::WhichFormula::ENDPOINT
-      stub_const("#{described_class}::DATABASE_FILE", test_db_file)
-
-      db = Homebrew::Cmd::WhichFormula::DATABASE_FILE
+      # Write the database where `brew which-formula` (a Bash command) reads it:
+      # the same path `Homebrew::API.write_executables_file!` writes to.
+      db = Homebrew::API::HOMEBREW_CACHE_API/"internal/executables.txt"
       db.dirname.mkpath
       db.write(<<~EOS)
         foo(1.0.0):foo2 foo3