Do not pass -safe-string explicitly

Strings are safe now.

examples/custom-utop/dune

@@ -1,5 +1,4 @@
 (executable
  (name myutop)
- (flags :standard -safe-string)
  (link_flags -linkall)
  (libraries utop))

examples/interact/dune

@@ -1,5 +1,4 @@
 (executable
  (name test_program)
- (flags :standard -safe-string)
  (link_flags -linkall)
  (libraries utop))

src/lib/dune

@@ -2,7 +2,6 @@
  (name uTop)
  (public_name utop)
  (wrapped false)
- (flags :standard -safe-string)
  (modes byte)
  (libraries compiler-libs.toplevel findlib.top lambda-term logs.lwt threads zed)
  (preprocess

src/top/dune

@@ -1,7 +1,6 @@
 (executables
  (names utop)
  (libraries utop)
- (flags :standard -safe-string)
  (modes byte)
  (link_flags -linkall))
 

src/top/expunge/dune

@@ -1,4 +1,3 @@
 (executable
  (name expunge)
- (flags :standard -safe-string)
  (libraries unix))