Match sidebar search clear button hover to the search dialog's

Drop the Chakra-era gray.100/gray.200 hover/active restatements and the
padding that widened the button: the plain ghost recipe (as used by
SearchDialog's clear button) gives the lighter blackAlpha hover, and the
square button's hover circle nests into the pill's rounded end.

Author: microbit-matt-hillsdon

src/workbench/SideBarHeader.tsx

@@ -268,18 +268,11 @@ const SideBarHeader = ({
               <IconButton
                 aria-label={intl.formatMessage({ id: "clear" })}
                 css={{
-                  backgroundColor: "white",
-                  // Restated: the background utility beats the ghost
-                  // variant's recipe-layer hover/active.
-                  _hover: { backgroundColor: "gray.100" },
-                  _active: { backgroundColor: "gray.200" },
                   // Also used for Zoom, move to theme.
                   color: "#838383",
                   fontSize: "2xl",
                   position: "absolute",
                   right: "0",
-                  pr: "3",
-                  pl: "3",
                 }}
                 onPress={handleClear}
                 variant="ghost"