omx: remove useless double negations

flush and port->flushing are both gboolean.
11 jobs for coding-style in 39 minutes and 52 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #553365
gst indent

00:00:15

passed #553364
manifest

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  Build
passed #553370
build clang fedora x86_64

00:14:00

passed #553366
build fedora x86_64

00:14:38

passed #553373
docker windows 1607
build msys2

00:15:03

passed #553367
build nodebug fedora x86_64

00:14:28

passed #553368
build static fedora x86_64

00:27:06

passed #553369
build static nodebug fedora x86_64

00:25:42

passed #553371
docker windows 1607
build vs2017 amd64

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passed #553372
docker windows 1607
build vs2017 x86

00:13:29

 
  Integrate
passed #553374
documentation

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