Writer: Delete writePreserve/writeAllPreserve

This is one way of partially addressing #24767

- These functions are unused
- These functions are untested
- These functions are broken
  + The same dangling pointer bug from 6219c01 exists in `writePreserve`
  + The order of the bytes preserved in relation to the `bytes` being written can differ depending on unused buffer capacity at the time of the call and the drain implementation.

If there ends up being a need for these functions, they can be fixed and added back.

Author: squeek502

lib/std/Io/Writer.zig

@@ -498,44 +498,13 @@ pub fn write(w: *Writer, bytes: []const u8) Error!usize {
     return w.vtable.drain(w, &.{bytes}, 1);
 }
 
-/// Asserts `buffer` capacity exceeds `preserve_len`.
-pub fn writePreserve(w: *Writer, preserve_len: usize, bytes: []const u8) Error!usize {
-    assert(preserve_len <= w.buffer.len);
-    if (w.end + bytes.len <= w.buffer.len) {
-        @branchHint(.likely);
-        @memcpy(w.buffer[w.end..][0..bytes.len], bytes);
-        w.end += bytes.len;
-        return bytes.len;
-    }
-    const temp_end = w.end -| preserve_len;
-    const preserved = w.buffer[temp_end..w.end];
-    w.end = temp_end;
-    defer w.end += preserved.len;
-    const n = try w.vtable.drain(w, &.{bytes}, 1);
-    assert(w.end <= temp_end + preserved.len);
-    @memmove(w.buffer[w.end..][0..preserved.len], preserved);
-    return n;
-}
-
 /// Calls `drain` as many times as necessary such that all of `bytes` are
 /// transferred.
 pub fn writeAll(w: *Writer, bytes: []const u8) Error!void {
     var index: usize = 0;
     while (index < bytes.len) index += try w.write(bytes[index..]);
 }
 
-/// Calls `drain` as many times as necessary such that all of `bytes` are
-/// transferred.
-///
-/// When draining the buffer, ensures that at least `preserve_len` bytes
-/// remain buffered.
-///
-/// Asserts `buffer` capacity exceeds `preserve_len`.
-pub fn writeAllPreserve(w: *Writer, preserve_len: usize, bytes: []const u8) Error!void {
-    var index: usize = 0;
-    while (index < bytes.len) index += try w.writePreserve(preserve_len, bytes[index..]);
-}
-
 /// Renders fmt string with args, calling `writer` with slices of bytes.
 /// If `writer` returns an error, the error is returned from `format` and
 /// `writer` is not called again.