Trac #34853: sage-env: Fix misconfiguration of pip

{{{
./sage -sh -c 'pip cache'
git:t/34851/upgrade_singular_to_4_3_1p3
An error occurred during configuration: option format: invalid choice:
'columns' (choose from 'human', 'abspath')
}}}
as noted in https://trac.sagemath.org/ticket/34850#comment:19

URL: https://trac.sagemath.org/34853
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

src/bin/sage-env

@@ -592,8 +592,6 @@ if [ "${MAKEFLAGS-__unset__}" != "__unset__" ]; then
 fi
 export MAKE
 
-export PIP_FORMAT="columns"
-
 # Set the cysignals crash logs directory
 if [ -z "$CYSIGNALS_CRASH_LOGS" ]; then
     export CYSIGNALS_CRASH_LOGS="$DOT_SAGE/crash_logs"