Do not ship contrib .gitignores

[na-Itms]

In bundled versions of Premake 5, .gitignores are included. The top-level one is not harmful (and might be needed if I understood correctly issue #595) but there are some of those files under contrib/ that can cause problems.

Namely, contrib/curl/include/curl/.gitignore excludes contrib/curl/include/curl/curlbuild.h which is needed to build premake5. When including the bundled version of premake in a git repository, this file is silently ignored and users stumble upon this kind of failure:

==== Building curl-lib (release) ====
asyn-thread.c
In file included from ../../contrib/curl/lib/asyn-thread.c:60:
In file included from ../../contrib/curl/lib/urldata.h:82:
In file included from ../../contrib/curl/lib/cookie.h:26:
../../contrib/curl/include/curl/curl.h:34:10: fatal error: 'curlbuild.h' file not found
#include "curlbuild.h"       /* libcurl build definitions */
         ^
1 error generated.

I think those third-party gitignore files should be removed from bundles.

contrib/curl/include/curl/.gitignore
contrib/curl/lib/.gitignore
contrib/mbedtls/.gitignore
contrib/mbedtls/{include,library,programs,test,yotta}/.gitignore