Remove test for executable DSO

This test fails on some Linux systems, and it is essentially testing
not the mold's capabilities but the running platform's capabilities.
I don't think there's a portable, guaranteed way to create an
executable shared object on Linux, so even if it happens to work,
I don't think we can guarantee that.

test/elf/executable-dso.sh

@@ -1,45 +0,0 @@
-#!/bin/bash
-export LC_ALL=C
-set -e
-CC="${CC:-cc}"
-CXX="${CXX:-c++}"
-GCC="${GCC:-gcc}"
-GXX="${GXX:-g++}"
-OBJDUMP="${OBJDUMP:-objdump}"
-MACHINE="${MACHINE:-$(uname -m)}"
-testname=$(basename "$0" .sh)
-echo -n "Testing $testname ... "
-cd "$(dirname "$0")"/../..
-mold="$(pwd)/mold"
-t=out/test/elf/$testname
-mkdir -p $t
-
-echo 'int main() {}' | $CC -o $t/exe -xc -
-dyld=$(readelf -Wl $t/exe | grep 'program interpreter:' | sed -e 's!.*: \(.*\)\]!\1!')
-
-cat <<EOF | $CC -o $t/a.o -c -xc -fPIC -
-#include <stdio.h>
-#include <stdlib.h>
-
-void hello() {
-  puts("Hello world");
-}
-
-void _start() {
-  hello();
-  exit(0);
-}
-EOF
-
-$CC -B. -o $t/b.so $t/a.o -shared -Wl,--dynamic-linker=$dyld
-$QEMU $t/b.so | grep -q 'Hello world'
-
-cat <<EOF | $CC -o $t/c.o -c -xc -
-void hello();
-int main() { hello(); }
-EOF
-
-$CC -B. -o $t/exe $t/c.o $t/b.so
-$QEMU $t/exe | grep -q 'Hello world'
-
-echo OK