Move BTF verbose messages to debug messages

Before, you'd get:
```
$ sudo ./build/src/bpftrace -lv "kfunc:hrtimers_dead_cpu"
BTF: using data from /sys/kernel/btf/vmlinux
kfunc:hrtimers_dead_cpu
    unsigned int scpu;
    int retval;
```

The first message is kind of ugly and not very actionable or informative
for users. Better to move it to debug where bpftrace developers might
actually make use of the information.

CHANGELOG.md

@@ -52,6 +52,8 @@ and this project adheres to
   - [#1542](https://github.com/iovisor/bpftrace/pull/1542)
 - Change a part of the message of '-v' output
   - [#1553](https://github.com/iovisor/bpftrace/pull/1553)
+- Remove "BTF: using data from ..." message when using -v flag
+  - [#1554](https://github.com/iovisor/bpftrace/pull/1554)
 
 #### Deprecated
 

docs/reference_guide.md

@@ -298,7 +298,6 @@ If BTF is available, it is also possible to list struct/union/emum definitions.
 
 ```
 # bpftrace -lv "struct path"
-BTF: using data from /sys/kernel/btf/vmlinux
 struct path {
         struct vfsmount *mnt;
         struct dentry *dentry;

src/btf.cpp

@@ -115,7 +115,7 @@ static struct btf *btf_open(const struct vmlinux_location *locs)
 
     if (err)
     {
-      if (bt_verbose)
+      if (bt_debug != DebugLevel::kNone)
       {
         char err_buf[256];
 
@@ -126,7 +126,7 @@ static struct btf *btf_open(const struct vmlinux_location *locs)
       continue;
     }
 
-    if (bt_verbose)
+    if (bt_debug != DebugLevel::kNone)
     {
       std::cerr << "BTF: using data from " << path << std::endl;
     }