lib3026: drop DLL pre-load perf mitigation for old mingw

lib3026: drop DLL pre-load perf mitigation for old mingw

curl no longer supports old/legacy/classic mingw.

This mitigation was addressing slow perf seen in CI with old mingw.
The slow perf is not seen in current CI with supported compilers.

Remove the duplicate DLL load function from libtest. It's no longer
used after this patch.

Current CI run times for test3026 on GHA/windows:
```
test 3026...[curl_global_init thread-safety]
 # mingw, CM clang-x86_64 gnutls libssh
 -------e--- OK (1715 out of 1738, remaining: 00:02, took 0.196s, duration: 02:55)
 # dl-mingw, CM 9.5.0-x86_64 schannel
 -------e--- OK (1554 out of 1577, remaining: 00:02, took 0.217s, duration: 02:29)
 # msvc, CM x64-windows schannel +examples
 -------e--- OK (1578 out of 1601, remaining: 00:02, took 0.205s, duration: 02:50)
```

Follow-up to 38029101e2d78ba125732b3bab6ec267b80a0e72 #11625
Follow-up to 856b133f5d62475d4cc12624c4cccb4170134712 #9412
Ref: #17413

Closes #17414