box: fix UBSan error regarding misaligned store in field_map.c

The type cast is unnecessary and causes false-positive errors:

NO_WRAP
```
./src/box/field_map.c:110:10: runtime error: store to misaligned address 0x507000071082 for type 'uint32_t *' (aka 'unsigned int *'), which requires 4 byte alignment
0x507000071082: note: pointer points here
 01 00  00 00 be be be be f0 ff  ff ff 02 00 00 00 be be  be be be be be be 00 00  00 00 00 00 00 00
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ./src/box/field_map.c:110:10
```
NO_WRAP

Closes #10631

NO_DOC=bugfix
NO_CHANGELOG=minor
NO_TEST=tested by debug_asan_clang workflow

(cherry picked from commit 5ddbd85cc377a29dc27d01ad06acdc6acc24cc5b)