Undefined reference to concurrency on Solaris

The MariaDB 10.11.6 `mysqld` doesn't link on Solaris:
```
Undefined first referenced
symbol in file
concurrency ../../sql/libsql.a(mysqld.cc.o)
```
This is from a call in `sql/mysqld.cc` (`mysql_main`):
```
(void) thr_setconcurrency(concurrency); // 10 by default
```
The `concurrency` variable had been ripped out in MDEV-18650 with the deprecation of `thread_concurrency`, but that has been incomplete. It seems that this issue wasn't noticed on other targets since `my_pthread.h` has:
```
#ifndef HAVE_THR_SETCONCURRENCY
#define thr_setconcurrency(A) pthread_dummy(0)
#endif
```
while Solaris *does* have `thr_setconcurrency`.

Given the previous (incomplete) removal, I've just removed this call as well.