t9300: remove unnecessary use of /dev/stdin

We really shouldn't be using these funny /dev/* files that did not exist
in V7 UNIX in our tests when we do not have to.

Output from

    $ git grep -n -e /dev/ --and --not -e /dev/null t/

tells us that, aside from use of /dev/urandom in apache.conf used in http
tests, "dd if=/dev/stdin" added recently to t/t9300-fast-import.sh are the
only offenders, and "dd" reads from the standard input by default, so
removing them should be straightforward.

Reported-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9300-fast-import.sh

@@ -1794,7 +1794,7 @@ test_expect_success PIPE 'R: copy using cat-file' '
 
 	read blob_id type size <&3 &&
 	echo "$blob_id $type $size" >response &&
-	dd if=/dev/stdin of=blob bs=$size count=1 <&3 &&
+	dd of=blob bs=$size count=1 <&3 &&
 	read newline <&3 &&
 
 	cat <<EOF &&
@@ -1845,7 +1845,7 @@ test_expect_success PIPE 'R: print blob mid-commit' '
 		EOF
 
 		read blob_id type size <&3 &&
-		dd if=/dev/stdin of=actual bs=$size count=1 <&3 &&
+		dd of=actual bs=$size count=1 <&3 &&
 		read newline <&3 &&
 
 		echo
@@ -1880,7 +1880,7 @@ test_expect_success PIPE 'R: print staged blob within commit' '
 		echo "cat-blob $to_get" &&
 
 		read blob_id type size <&3 &&
-		dd if=/dev/stdin of=actual bs=$size count=1 <&3 &&
+		dd of=actual bs=$size count=1 <&3 &&
 		read newline <&3 &&
 
 		echo deleteall