Removed ipc module.

There was only one function in module ipc: eintr_retry_call. This
function was used in pdftohtml module, and turns out[1] it's not needed
anymore. Support for retrying on syscall interrupts was introduced in
Python 3.5 (which was released in 2015), and covers subprocess module
among other modules. In this commit, mentioned eintr_retry_call function
has been removed, and couple of cosmetic changes was done in module
pdftohtml.

[1] https://www.python.org/dev/peps/pep-0475/

ebook_converter/utils/ipc/__init__.py

@@ -1,11 +0,0 @@
-import errno
-
-
-def eintr_retry_call(func, *args, **kwargs):
-    while True:
-        try:
-            return func(*args, **kwargs)
-        except EnvironmentError as e:
-            if getattr(e, 'errno', None) == errno.EINTR:
-                continue
-            raise