gccrs: Fix multiple glob imports in a use decl

[powerboat9]

Also removes some dead code in flatten, since I saw it while working on this PR and it's too small to be its own patch.

Fixes #4702

[powerboat9]

Seems to pair us down to

./test-rust/rust/library/core/src/iter/adapters/chain.rs:211:33: error: identifier ‘None’ is bound more than once in the same pattern [E0416]
  211 |             Chain { a: None, b: None } => (0, Some(0)),
      |                                 ^~~~
./test-rust/rust/library/core/src/iter/adapters/zip.rs:153:20: error: identifier ‘None’ is bound more than once in the same pattern [E0416]
  153 |             (None, None) => None,
      |                    ^~~~
./test-rust/rust/library/core/src/iter/adapters/zip.rs:169:20: error: identifier ‘None’ is bound more than once in the same pattern [E0416]
  169 |             (None, None) => None,
      |                    ^~~~
./test-rust/rust/library/core/src/iter/traits/iterator.rs:3237:46: error: variable ‘None’ is not bound in all patterns [E0408]
 3237 |             if let Some(Ordering::Greater) | None = compare(&last, &curr) {
      |                                              ^~~~
./test-rust/rust/library/core/src/slice/iter.rs:1974:3: error: could not resolve type path ‘N’ [E0412]
 1974 | #[derive(Debug, Clone, Copy)]
      |   ^~~~~~
./test-rust/rust/library/core/src/slice/iter.rs:1974:3: error: could not resolve type path ‘N’ [E0412]
./test-rust/rust/library/core/src/slice/iter.rs:1974:3: error: could not resolve type path ‘N’ [E0412]
./test-rust/rust/library/core/src/slice/iter.rs:2096:3: error: could not resolve type path ‘N’ [E0412]
 2096 | #[derive(Debug)]
      |   ^~~~~~
./test-rust/rust/library/core/src/slice/iter.rs:2218:3: error: could not resolve type path ‘N’ [E0412]
 2218 | #[derive(Debug)]
      |   ^~~~~~

for compiling core 1.49 (not including errors detected in later phases, of course).