Remove repeat and enforce-determinism options

These only functioned if a very narrow combination of conditions held:

- The result path does not yet exist (--check did not result in
  repeated builds), AND
- The result path is not available from any configured substituters, AND
- No remote builders that can build the path are available.

If any of these do not hold, a derivation would be built 0 or 1 times
regardless of the repeat option. Thus, remove it to avoid confusion.

doc/manual/src/command-ref/nix-store.md

@@ -104,10 +104,6 @@ The following flags are available:
     previous build, the new output path is left in
     `/nix/store/name.check.`
 
-    See also the `build-repeat` configuration option, which repeats a
-    derivation a number of times and prevents its outputs from being
-    registered as “valid” in the Nix store unless they are identical.
-
 Special exit codes:
 
   - `100`\