common: remove references to non-existent GGML_SCHED_PRIO_LOW

The GGML_SCHED_PRIO_LOW enumeration value doesn't exist in the current
ggml API. The available priority levels are:
- GGML_SCHED_PRIO_NORMAL
- GGML_SCHED_PRIO_MEDIUM
- GGML_SCHED_PRIO_HIGH
- GGML_SCHED_PRIO_REALTIME

Remove all references to GGML_SCHED_PRIO_LOW in the priority handling
code and update the validation range checking accordingly.

This fixes compilation errors when building against system-provided
ggml libraries.

Author: mastered-lore

common/arg.cpp

@@ -1350,7 +1350,7 @@ common_params_context common_params_parser_init(common_params & params, llama_ex
         {"--prio"}, "N",
         string_format("set process/thread priority : low(-1), normal(0), medium(1), high(2), realtime(3) (default: %d)\n", params.cpuparams.priority),
         [](common_params & params, int prio) {
-            if (prio < GGML_SCHED_PRIO_LOW || prio > GGML_SCHED_PRIO_REALTIME) {
+            if (prio < GGML_SCHED_PRIO_NORMAL || prio > GGML_SCHED_PRIO_REALTIME) {
                 throw std::invalid_argument("invalid value");
             }
             params.cpuparams.priority = (enum ggml_sched_priority) prio;

common/common.cpp

@@ -203,7 +203,6 @@ bool set_process_priority(enum ggml_sched_priority prio) {
 
     DWORD p = NORMAL_PRIORITY_CLASS;
     switch (prio) {
-        case GGML_SCHED_PRIO_LOW:      p = BELOW_NORMAL_PRIORITY_CLASS; break;
         case GGML_SCHED_PRIO_NORMAL:   p = NORMAL_PRIORITY_CLASS;       break;
         case GGML_SCHED_PRIO_MEDIUM:   p = ABOVE_NORMAL_PRIORITY_CLASS; break;
         case GGML_SCHED_PRIO_HIGH:     p = HIGH_PRIORITY_CLASS;         break;
@@ -229,7 +228,6 @@ bool set_process_priority(enum ggml_sched_priority prio) {
 
     int p = 0;
     switch (prio) {
-        case GGML_SCHED_PRIO_LOW:      p =  5;  break;
         case GGML_SCHED_PRIO_NORMAL:   p =  0;  break;
         case GGML_SCHED_PRIO_MEDIUM:   p = -5;  break;
         case GGML_SCHED_PRIO_HIGH:     p = -10; break;