tests (mingw): remove Bash-specific pwd option

The -W option is only understood by MSYS2 Bash's pwd command. We already
make sure to override `pwd` by `builtin pwd -W` for MINGW, so let's not
double the effort here.

This will also help when switching the shell to another one (such as
BusyBox' ash) whose pwd does *not* understand the -W option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t9902-completion.sh

@@ -139,12 +139,7 @@ invalid_variable_name='${foo.bar}'
 
 actual="$TRASH_DIRECTORY/actual"
 
-if test_have_prereq MINGW
-then
-	ROOT="$(pwd -W)"
-else
-	ROOT="$(pwd)"
-fi
+ROOT="$(pwd)"
 
 test_expect_success 'setup for __git_find_repo_path/__gitdir tests' '
 	mkdir -p subdir/subsubdir &&