example/python: avoid falling back to CLOCK_MONOTONIC

According to the D-Bus API specification we return CLOCK_BOOTTIME only.
We don't support kernels too old to have it -- the fall back to
CLOCK_MONOTONIC is only there to be able to run unit tests on RHEL 6
kernel and will eventually go away.

examples/python/nmex.py

@@ -50,18 +50,8 @@ def sys_clock_gettime_ns(clock_id):
     return _sys_clock_gettime_ns(clock_id)
 
 def nm_boot_time_ns():
-    # NetworkManager exposes some timestamps as CLOCK_BOOTTIME.
+    CLOCK_BOOTTIME = 7
-    # Try that first (number 7).
+    return sys_clock_gettime_ns(CLOCK_BOOTTIME)
-    try:
-        return sys_clock_gettime_ns(7)
-    except OSError as e:
-        # On systems, where this is not available, fallback to
-        # CLOCK_MONOTONIC (numeric 1).
-        # That is what NetworkManager does as well.
-        import errno
-        if e.errno == errno.EINVAL:
-            return sys_clock_gettime_ns(1)
-        raise
 def nm_boot_time_us():
     return nm_boot_time_ns() / 1000
 def nm_boot_time_ms():