[reggen] Remove stray import

This is unused since we removed fusesoc_core_name support and causes a
flake8 lint error.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/util/reggen/ip_block.py b/util/reggen/ip_block.py
index 4707922..9ff7786 100644
--- a/util/reggen/ip_block.py
+++ b/util/reggen/ip_block.py
@@ -11,7 +11,7 @@
 from .alert import Alert
 from .bus_interfaces import BusInterfaces
 from .inter_signal import InterSignal
-from .lib import (check_keys, check_name, check_int, check_str, check_bool,
+from .lib import (check_keys, check_name, check_int, check_bool,
                   check_list, check_optional_str, check_name_list)
 from .params import ReggenParams, LocalParam
 from .reg_block import RegBlock