Remove main.cpp from dolphinstatic_SRCS

Summary:
The file main.cpp is already in dolphin_SRCS and doesn't
belong in dolphinstatic_SRCS.

Normally the duplicate object is simply ignored, but with
link time optimization (LTO), linking dolphin can fail.
Apparently, the compiler tries to inline inline kdemain()
in this case. That is undesirable anyway and it ultimately
fails because the DBusInterface definition is not available:

.../ccHEv6cl.ltrans0.ltrans.o: In function `DBusInterface::~DBusInterface()':
<artificial>:(.text+0x2583): undefined reference to `vtable for DBusInterface'
.../ccHEv6cl.ltrans0.ltrans.o: In function `DBusInterface::~DBusInterface()':
<artificial>:(.text+0x5aa3): undefined reference to `vtable for DBusInterface'
.../ccHEv6cl.ltrans0.ltrans.o: In function `kdemain':
<artificial>:(.text+0x7686): undefined reference to `DBusInterface::DBusInterface()'
<artificial>:(.text+0x7b64): undefined reference to `vtable for DBusInterface'
collect2: error: ld returned 1 exit status

See also <https://bugs.gentoo.org/655710>.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, asturmlechner, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D12929