Fix flake8 "invalid escape sequence 'x'" warning

From, https://lintlyci.github.io/Flake8Rules/rules/W605.html

As of Python 3.6, a backslash-character pair that is not a valid
escape sequence now generates a DeprecationWarning. Although this
will eventually become a SyntaxError, that will not be for several
Python releases.

Note 'float_pobj' was simply removed from arcstat.py since it
was entirely unused.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8056

cmd/arcstat/arcstat.py

@@ -112,7 +112,6 @@ cur = {}
 d = {}
 out = None
 kstat = None
-float_pobj = re.compile("^[0-9]+(\.[0-9]+)?$")
 
 
 def detailed_usage():